- 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>
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