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
19ab3b5656