feat(story-200): auto-prune worktrees when stories are archived

- Add `prune_worktree_sync` to worktree.rs: removes a story's worktree
  if it exists, delegating to `remove_worktree_sync` (best-effort,
  failures logged internally)
- Update `sweep_done_to_archived` to accept `git_root` and call
  `prune_worktree_sync` after promoting a story from 5_done to 6_archived
- Add Part 2 to the sweep: scan 6_archived and prune any stale worktrees
  for stories already there (catches items archived before this feature)
- All worktree removal failures are logged but never block file moves
- Add 5 new tests: prune noop, prune real worktree, sweep-on-promote,
  sweep-stale-archived, sweep-not-blocked-by-removal-failure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/io/watcher.rs

@@ -206,59 +206,89 @@ fn flush_pending(
 }
 
 /// Scan `work/5_done/` and move any `.md` files whose mtime is older than
-/// `done_retention` to `work/6_archived/`.
+/// `done_retention` to `work/6_archived/`. After each successful promotion,
+/// removes the associated git worktree (if any) via [`crate::worktree::prune_worktree_sync`].
+///
+/// Also scans `work/6_archived/` for stories that still have a live worktree
+/// and removes them (catches items that were archived before this sweep was
+/// added).
+///
+/// Worktree removal failures are logged but never block the file move or other
+/// cleanup work.
 ///
 /// Called periodically from the watcher thread. File moves will trigger normal
 /// watcher events, which `flush_pending` will commit and broadcast.
-fn sweep_done_to_archived(work_dir: &Path, done_retention: Duration) {
+fn sweep_done_to_archived(work_dir: &Path, git_root: &Path, done_retention: Duration) {
 
+    // ── Part 1: promote old items from 5_done/ → 6_archived/ ───────────────
     let done_dir = work_dir.join("5_done");
-    if !done_dir.exists() {
-        return;
+    if done_dir.exists() {
+        let archived_dir = work_dir.join("6_archived");
+
+        match std::fs::read_dir(&done_dir) {
+            Err(e) => slog!("[watcher] sweep: failed to read 5_done/: {e}"),
+            Ok(entries) => {
+                for entry in entries.flatten() {
+                    let path = entry.path();
+                    if path.extension().is_none_or(|e| e != "md") {
+                        continue;
+                    }
+
+                    let mtime = match entry.metadata().and_then(|m| m.modified()) {
+                        Ok(t) => t,
+                        Err(_) => continue,
+                    };
+
+                    let age = SystemTime::now()
+                        .duration_since(mtime)
+                        .unwrap_or_default();
+
+                    if age >= done_retention {
+                        if let Err(e) = std::fs::create_dir_all(&archived_dir) {
+                            slog!("[watcher] sweep: failed to create 6_archived/: {e}");
+                            continue;
+                        }
+                        let dest = archived_dir.join(entry.file_name());
+                        match std::fs::rename(&path, &dest) {
+                            Ok(()) => {
+                                let item_id = path
+                                    .file_stem()
+                                    .and_then(|s| s.to_str())
+                                    .unwrap_or("unknown");
+                                slog!("[watcher] sweep: promoted {item_id} → 6_archived/");
+                                // Prune the worktree for this story (best effort).
+                                if let Err(e) =
+                                    crate::worktree::prune_worktree_sync(git_root, item_id)
+                                {
+                                    slog!(
+                                        "[watcher] sweep: worktree prune failed for {item_id}: {e}"
+                                    );
+                                }
+                            }
+                            Err(e) => {
+                                slog!("[watcher] sweep: failed to move {}: {e}", path.display());
+                            }
+                        }
+                    }
+                }
+            }
+        }
     }
 
-    let entries = match std::fs::read_dir(&done_dir) {
-        Ok(e) => e,
-        Err(e) => {
-            slog!("[watcher] sweep: failed to read 5_done/: {e}");
-            return;
-        }
-    };
-
+    // ── Part 2: prune stale worktrees for items already in 6_archived/ ──────
     let archived_dir = work_dir.join("6_archived");
-
-    for entry in entries.flatten() {
-        let path = entry.path();
-        if path.extension().is_none_or(|e| e != "md") {
-            continue;
-        }
-
-        let mtime = match entry.metadata().and_then(|m| m.modified()) {
-            Ok(t) => t,
-            Err(_) => continue,
-        };
-
-        let age = SystemTime::now()
-            .duration_since(mtime)
-            .unwrap_or_default();
-
-        if age >= done_retention {
-            if let Err(e) = std::fs::create_dir_all(&archived_dir) {
-                slog!("[watcher] sweep: failed to create 6_archived/: {e}");
+    if archived_dir.exists()
+        && let Ok(entries) = std::fs::read_dir(&archived_dir)
+    {
+        for entry in entries.flatten() {
+            let path = entry.path();
+            if path.extension().is_none_or(|e| e != "md") {
                 continue;
             }
-            let dest = archived_dir.join(entry.file_name());
-            match std::fs::rename(&path, &dest) {
-                Ok(()) => {
-                    let item_id = path
-                        .file_stem()
-                        .and_then(|s| s.to_str())
-                        .unwrap_or("unknown");
-                    slog!("[watcher] sweep: promoted {item_id} → 6_archived/");
-                }
-                Err(e) => {
-                    slog!("[watcher] sweep: failed to move {}: {e}", path.display());
-                }
+            if let Some(item_id) = path.file_stem().and_then(|s| s.to_str())
+                && let Err(e) = crate::worktree::prune_worktree_sync(git_root, item_id)
+            {
+                slog!("[watcher] sweep: worktree prune failed for {item_id}: {e}");
             }
         }
     }
@@ -410,7 +440,7 @@ pub fn start_watcher(
                 let now = Instant::now();
                 if now.duration_since(last_sweep) >= sweep_interval {
                     last_sweep = now;
-                    sweep_done_to_archived(&work_dir, done_retention);
+                    sweep_done_to_archived(&work_dir, &git_root, done_retention);
                 }
             }
         }
@@ -758,7 +788,8 @@ mod tests {
             .unwrap();
 
         let retention = Duration::from_secs(4 * 60 * 60);
-        sweep_done_to_archived(&work_dir, retention);
+        // tmp.path() has no worktrees dir — prune_worktree_sync is a no-op.
+        sweep_done_to_archived(&work_dir, tmp.path(), retention);
 
         assert!(!story_path.exists(), "old item should be moved out of 5_done/");
         assert!(
@@ -779,7 +810,7 @@ mod tests {
         fs::write(&story_path, "---\nname: new\n---\n").unwrap();
 
         let retention = Duration::from_secs(4 * 60 * 60);
-        sweep_done_to_archived(&work_dir, retention);
+        sweep_done_to_archived(&work_dir, tmp.path(), retention);
 
         assert!(story_path.exists(), "recent item should remain in 5_done/");
     }
@@ -802,7 +833,7 @@ mod tests {
             .unwrap();
 
         // With a 1-minute retention, the 2-minute-old file should be swept.
-        sweep_done_to_archived(&work_dir, Duration::from_secs(60));
+        sweep_done_to_archived(&work_dir, tmp.path(), Duration::from_secs(60));
 
         assert!(
             !story_path.exists(),
@@ -831,11 +862,142 @@ mod tests {
             .unwrap();
 
         // With a 1-minute retention, the 30-second-old file should stay.
-        sweep_done_to_archived(&work_dir, Duration::from_secs(60));
+        sweep_done_to_archived(&work_dir, tmp.path(), Duration::from_secs(60));
 
         assert!(
             story_path.exists(),
             "item younger than custom retention should remain"
         );
     }
+
+    // ── sweep worktree pruning ─────────────────────────────────────────────
+
+    /// Helper: create a real git worktree at `wt_path` on a new branch.
+    fn create_git_worktree(git_root: &std::path::Path, wt_path: &std::path::Path, branch: &str) {
+        use std::process::Command;
+        // Create the branch first (ignore errors if it already exists).
+        let _ = Command::new("git")
+            .args(["branch", branch])
+            .current_dir(git_root)
+            .output();
+        Command::new("git")
+            .args(["worktree", "add", &wt_path.to_string_lossy(), branch])
+            .current_dir(git_root)
+            .output()
+            .expect("git worktree add");
+    }
+
+    #[test]
+    fn sweep_prunes_worktree_when_story_promoted_to_archived() {
+        let tmp = TempDir::new().unwrap();
+        let git_root = tmp.path().to_path_buf();
+        init_git_repo(&git_root);
+
+        let work_dir = git_root.join(".story_kit").join("work");
+        let done_dir = work_dir.join("5_done");
+        fs::create_dir_all(&done_dir).unwrap();
+
+        let story_id = "60_story_prune_on_promote";
+        let story_path = done_dir.join(format!("{story_id}.md"));
+        fs::write(&story_path, "---\nname: test\n---\n").unwrap();
+        let past = SystemTime::now()
+            .checked_sub(Duration::from_secs(5 * 60 * 60))
+            .unwrap();
+        filetime::set_file_mtime(&story_path, filetime::FileTime::from_system_time(past))
+            .unwrap();
+
+        // Create a real git worktree for this story.
+        let wt_path = crate::worktree::worktree_path(&git_root, story_id);
+        fs::create_dir_all(wt_path.parent().unwrap()).unwrap();
+        create_git_worktree(&git_root, &wt_path, &format!("feature/story-{story_id}"));
+        assert!(wt_path.exists(), "worktree must exist before sweep");
+
+        let retention = Duration::from_secs(4 * 60 * 60);
+        sweep_done_to_archived(&work_dir, &git_root, retention);
+
+        // Story must be archived.
+        assert!(
+            !story_path.exists(),
+            "story should be moved out of 5_done/"
+        );
+        assert!(
+            work_dir.join("6_archived").join(format!("{story_id}.md")).exists(),
+            "story should appear in 6_archived/"
+        );
+        // Worktree must be removed.
+        assert!(!wt_path.exists(), "worktree should be removed after archiving");
+    }
+
+    #[test]
+    fn sweep_prunes_worktrees_for_already_archived_stories() {
+        let tmp = TempDir::new().unwrap();
+        let git_root = tmp.path().to_path_buf();
+        init_git_repo(&git_root);
+
+        let work_dir = git_root.join(".story_kit").join("work");
+        let archived_dir = work_dir.join("6_archived");
+        fs::create_dir_all(&archived_dir).unwrap();
+
+        // Story is already in 6_archived.
+        let story_id = "61_story_stale_worktree";
+        let story_path = archived_dir.join(format!("{story_id}.md"));
+        fs::write(&story_path, "---\nname: stale\n---\n").unwrap();
+
+        // Create a real git worktree that was never cleaned up.
+        let wt_path = crate::worktree::worktree_path(&git_root, story_id);
+        fs::create_dir_all(wt_path.parent().unwrap()).unwrap();
+        create_git_worktree(&git_root, &wt_path, &format!("feature/story-{story_id}"));
+        assert!(wt_path.exists(), "stale worktree must exist before sweep");
+
+        // 5_done/ is empty — only Part 2 runs.
+        fs::create_dir_all(work_dir.join("5_done")).unwrap();
+        let retention = Duration::from_secs(4 * 60 * 60);
+        sweep_done_to_archived(&work_dir, &git_root, retention);
+
+        // Stale worktree should be pruned.
+        assert!(!wt_path.exists(), "stale worktree should be pruned by sweep");
+        // Story file must remain untouched.
+        assert!(story_path.exists(), "archived story file must not be removed");
+    }
+
+    #[test]
+    fn sweep_archives_story_even_when_worktree_removal_fails() {
+        // Use a git repo so prune_worktree_sync can attempt removal,
+        // but the fake directory is not a registered git worktree so
+        // `git worktree remove` will fail — the story must still be archived.
+        let tmp = TempDir::new().unwrap();
+        let git_root = tmp.path().to_path_buf();
+        init_git_repo(&git_root);
+
+        let work_dir = git_root.join(".story_kit").join("work");
+        let done_dir = work_dir.join("5_done");
+        fs::create_dir_all(&done_dir).unwrap();
+
+        let story_id = "62_story_fake_worktree";
+        let story_path = done_dir.join(format!("{story_id}.md"));
+        fs::write(&story_path, "---\nname: test\n---\n").unwrap();
+        let past = SystemTime::now()
+            .checked_sub(Duration::from_secs(5 * 60 * 60))
+            .unwrap();
+        filetime::set_file_mtime(&story_path, filetime::FileTime::from_system_time(past))
+            .unwrap();
+
+        // Create a plain directory at the expected worktree path — not a real
+        // git worktree, so `git worktree remove` will fail.
+        let wt_path = crate::worktree::worktree_path(&git_root, story_id);
+        fs::create_dir_all(&wt_path).unwrap();
+
+        let retention = Duration::from_secs(4 * 60 * 60);
+        sweep_done_to_archived(&work_dir, &git_root, retention);
+
+        // Story must still be archived despite the worktree removal failure.
+        assert!(
+            !story_path.exists(),
+            "story should be archived even when worktree removal fails"
+        );
+        assert!(
+            work_dir.join("6_archived").join(format!("{story_id}.md")).exists(),
+            "story should appear in 6_archived/ despite worktree removal failure"
+        );
+    }
 }