fix(#2853): sync .sisyphus state from worktree to main repo before removal

When .sisyphus/ is gitignored, task state written during worktree execution
is lost when the worktree is removed. Fix:

- add worktree-sync.ts: syncSisyphusStateFromWorktree() copies .sisyphus/
  contents from worktree to main repo directory
- update start-work.ts template: documents the sync step as CRITICAL when
  worktree_path is set in boulder.json
- update work-with-pr/SKILL.md: adds explicit sync step before worktree removal
- export from boulder-state index
- test: 5 scenarios covering no-.sisyphus, nested dirs, overwrite stale state

.opencode/skills/work-with-pr/SKILL.md

@@ -282,6 +282,18 @@ Once all three gates pass:
 gh pr merge "$PR_NUMBER" --squash --delete-branch
 ```
 
+### Sync .sisyphus state back to main repo
+
+Before removing the worktree, copy `.sisyphus/` state back. When `.sisyphus/` is gitignored, files written there during worktree execution are not committed or merged — they would be lost on worktree removal.
+
+```bash
+# Sync .sisyphus state from worktree to main repo (preserves task state, plans, notepads)
+if [ -d "$WORKTREE_PATH/.sisyphus" ]; then
+  mkdir -p "$ORIGINAL_DIR/.sisyphus"
+  cp -r "$WORKTREE_PATH/.sisyphus/"* "$ORIGINAL_DIR/.sisyphus/" 2>/dev/null || true
+fi
+```
+
 ### Clean up the worktree
 
 The worktree served its purpose — remove it to avoid disk bloat: