Accept story 45: Deterministic Story Lifecycle Management

- accept_story MCP tool moves current/ to archived/
- move_story_to_archived helper with idempotent behavior
- start_agent auto-moves upcoming/ to current/

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

server/src/agents.rs

@@ -652,6 +652,36 @@ impl AgentPool {
     }
 }
 
+/// Move a story file from current/ to archived/ (human accept action).
+///
+/// * If the story is in current/, it is renamed to archived/.
+/// * If the story is already in archived/, this is a no-op (idempotent).
+/// * If the story is not found in current/ or archived/, an error is returned.
+pub fn move_story_to_archived(project_root: &Path, story_id: &str) -> Result<(), String> {
+    let stories_dir = project_root.join(".story_kit").join("stories");
+    let current_path = stories_dir.join("current").join(format!("{story_id}.md"));
+    let archived_path = stories_dir.join("archived").join(format!("{story_id}.md"));
+
+    if archived_path.exists() {
+        // Already archived — idempotent, nothing to do.
+        return Ok(());
+    }
+
+    if current_path.exists() {
+        let archived_dir = stories_dir.join("archived");
+        std::fs::create_dir_all(&archived_dir)
+            .map_err(|e| format!("Failed to create archived stories directory: {e}"))?;
+        std::fs::rename(&current_path, &archived_path)
+            .map_err(|e| format!("Failed to move story '{story_id}' to archived/: {e}"))?;
+        eprintln!("[lifecycle] Moved story '{story_id}' from current/ to archived/");
+        return Ok(());
+    }
+
+    Err(format!(
+        "Story '{story_id}' not found in current/. Cannot accept story."
+    ))
+}
+
 // ── Acceptance-gate helpers ───────────────────────────────────────────────────
 
 /// Check whether the given directory has any uncommitted git changes.