[chore] Add merged branch and worktree cleanup script

[ThreeLight2000]

Motivation

After a pull request is merged, its local feature branch and linked worktree can remain behind. Manually checking and removing them is repetitive and makes it easy to delete the wrong branch or overlook stale worktree registrations.

Scope

• Add a root pnpm script for cleanup.
• Detect local branches merged into the selected mainline (prefer origin/main, with a local fallback).
• Show candidates before changing anything and require confirmation by default.
• Protect the current and base branches.
• Remove eligible linked worktrees before deleting their local branches.
• Support dry-run, non-interactive confirmation, and explicit handling of dirty worktrees.
• Out of scope: deleting remote branches automatically.

Acceptance criteria

• A pnpm command lists merged local branches and linked worktrees before cleanup.
• The default flow asks for confirmation and does not remove the current or base branch.
• Eligible clean worktrees are removed before their merged local branches.
• Dirty worktrees are skipped unless an explicit force option is supplied.
• Dry-run mode makes no repository changes.
• Missing worktree registrations can be pruned safely.

Verification plan

1. Run pnpm cleanup:merged -- --dry-run.
2. Verify the candidate list and protected branches against git worktree list and git branch --merged.
3. Run the script on a disposable merged branch/worktree and verify both are removed.
4. Run the repository's relevant lint/type checks.

[ThreeLight2000]

Implementation started on the chore/16-merged-worktree-cleanup branch. The initial scope covers the root pnpm cleanup:merged command with dry-run support, protected branches, linked-worktree cleanup, dirty-worktree safety, and stale-registration pruning. Verification will include a dry-run and disposable Git worktree checks.

[ThreeLight2000]

Implementation and verification complete. Added the root pnpm cleanup:merged command and verified help output, dry-run behavior, base-ref fallback, protected branches, non-interactive safety, clean/dirty linked-worktree handling, and pnpm verify. The issue remains open for the pull request and review.

[ThreeLight2000]

작업을 시작합니다. chore/16-merged-worktree-cleanup 브랜치에서 pnpm cleanup:merged 스크립트의 병합 브랜치·연결 worktree 정리 동작과 안전장치(dry-run, 확인, dirty worktree 보호)를 구현하고 검증하겠습니다.

[ThreeLight2000]

구현 및 로컬 검증을 완료했습니다.

• scripts/cleanup-merged.mjs 추가 및 pnpm cleanup:merged 명령 연결
• 병합된 로컬 브랜치와 clean linked worktree 정리
• 현재 브랜치와 merge base 보호
• dirty/확인 프롬프트/--dry-run/--force/stale worktree --no-prune 안전장치 지원
• CLI 테스트 4개 통과
• pnpm verify 통과

변경사항은 chore/16-merged-worktree-cleanup 로컬 브랜치에 있으며, PR은 아직 생성하지 않았습니다.

[ThreeLight2000]

PR을 생성했습니다: #21

• Draft PR: [chore] Add merged branch and worktree cleanup script (#16)
• type:chore 라벨 적용
• Fixes #16 연결
• 변경 파일 3개, 로컬 검증 완료