Git worktree cleanup now confirms the repository first

A git worktree is an extra working folder attached to the same repository. Claude Code can clean these up.

It used to run git worktree remove --force directly. It now points git at a checked --git-dir when listing and removing worktrees. If it cannot confirm that git directory, it does not run git and says so. Git's records of worktrees whose folders no longer exist are now removed separately.