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/worktree.rs

@@ -166,6 +166,21 @@ fn configure_sparse_checkout(_wt_path: &Path) -> Result<(), String> {
     Ok(())
 }
 
+/// Remove the git worktree for a story if it exists, deriving the path and
+/// branch name deterministically from `project_root` and `story_id`.
+///
+/// Returns `Ok(())` if the worktree was removed or did not exist.
+/// Removal is best-effort: `remove_worktree_sync` logs failures internally
+/// but always returns `Ok`.
+pub fn prune_worktree_sync(project_root: &Path, story_id: &str) -> Result<(), String> {
+    let wt_path = worktree_path(project_root, story_id);
+    if !wt_path.exists() {
+        return Ok(());
+    }
+    let branch = branch_name(story_id);
+    remove_worktree_sync(project_root, &wt_path, &branch)
+}
+
 /// Remove a git worktree and its branch.
 pub async fn remove_worktree(
     project_root: &Path,
@@ -653,6 +668,33 @@ mod tests {
         assert!(result.is_ok(), "Expected removal to succeed: {:?}", result.err());
     }
 
+    // ── prune_worktree_sync ──────────────────────────────────────────────────
+
+    #[test]
+    fn prune_worktree_sync_noop_when_no_worktree_dir() {
+        let tmp = TempDir::new().unwrap();
+        // No worktree directory exists — must return Ok without touching git.
+        let result = prune_worktree_sync(tmp.path(), "42_story_nonexistent");
+        assert!(result.is_ok(), "Expected Ok when worktree dir absent: {:?}", result.err());
+    }
+
+    #[test]
+    fn prune_worktree_sync_removes_real_worktree() {
+        let tmp = TempDir::new().unwrap();
+        let project_root = tmp.path().join("my-project");
+        fs::create_dir_all(&project_root).unwrap();
+        init_git_repo(&project_root);
+
+        let story_id = "55_story_prune_test";
+        let wt_path = worktree_path(&project_root, story_id);
+        create_worktree_sync(&project_root, &wt_path, &format!("feature/story-{story_id}")).unwrap();
+        assert!(wt_path.exists(), "worktree dir should exist before prune");
+
+        let result = prune_worktree_sync(&project_root, story_id);
+        assert!(result.is_ok(), "prune_worktree_sync must return Ok: {:?}", result.err());
+        assert!(!wt_path.exists(), "worktree dir should be gone after prune");
+    }
+
     #[tokio::test]
     async fn create_worktree_succeeds_despite_setup_failure() {
         let tmp = TempDir::new().unwrap();