feat(worktree): delete branches after merged cleanup

[christso]

Summary

• add remove --merged for post-merge cleanup of both the linked worktree and its local branch
• preserve normal removal for unfinished branches and existing dirty-worktree protection
• resolve the checked-out branch before removal and delete it through the shared Git directory, including after squash merges
• document the explicit merge-confirmation contract and completion checks
• add real temporary-repository regression coverage

Verification

• bun test tests/unit/scripts/worktree.test.ts — 2 passed
• bash -n plugins/engineering/skills/worktree/scripts/worktree.sh
• bun run lint
• bun run typecheck
• bun test — 1462 passed, 5 skipped, 0 failed
• final code review — no PR-blocking findings

Manual E2E

1. Initialized a temporary Git repository on main and configured repository-local test identity.
2. Ran the packaged script with add feat/merged main and a temporary WORKTREE_ROOT.
3. Committed a feature file in the linked worktree.
4. Ran git merge --squash feat/merged and committed the squash on main.
5. Confirmed git merge-base --is-ancestor feat/merged main returned nonzero, exercising the squash-merge case rather than ordinary ancestry.
6. Ran remove --merged feat/merged through the packaged script.
7. Confirmed the worktree directory was absent, git worktree list --porcelain no longer registered it, and git branch --list feat/merged printed nothing.