Stale worktrees are never cleaned up

Problem

Claude Code creates git worktrees under ~/.claude-worktrees/ for parallel task execution but has no mechanism to clean them up if a session ends without proper cleanup (crash, interruption, or bug in teardown). There is also no garbage collection on subsequent session starts — each session is completely unaware of what previous sessions left behind.

This leaves behind:

1. Full repo copies on disk (wasting space)
2. Git branches that cannot be deleted even with git branch -D because git refuses to delete a branch checked out in any worktree
3. Worktree refs in .git/worktrees/ that persist indefinitely

The only way a user discovers these is by stumbling into the cannot delete branch error, at which point they have to manually git worktree remove or rm -rf the directory, then git worktree prune to clean up refs.

Reproduction

1. Use Claude Code on a project where it creates a worktree (parallel task execution)
2. End the session (or have it crash/interrupt)
3. Observe that ~/.claude-worktrees/<project>/<worktree-name> persists
4. Try to delete the branch: git branch -D <worktree-name> → fails with error: cannot delete branch used by worktree

Expected behavior

• Claude Code should clean up worktrees on session exit
• On session start, Claude Code should scan for and prune stale worktrees from previous sessions
• At minimum, a claude --cleanup command or similar should exist

Environment

• macOS (Darwin 25.2.0)
• Claude Code (CLI)
• Found a stale worktree from January 15, 2026 that was never cleaned up across dozens of subsequent sessions