'already used by worktree at <deleted path>': run git worktree prune

If an agent's worktree directory is removed with rm -rf (cleanup scripts, tmp reaping, a crashed harness) instead of git worktree remove, git keeps the admin entry in .git/worktrees/<name> and still treats its branch as checked out. Both git checkout b1 and git worktree add ../w2 b1 then fail with "fatal: 'b1' is already used by worktree at '/path/that/no/longer/exists'" (git 2.43.0). git worktree prune drops entries whose directory is gone, and after it the branch can be checked out again. git worktree list marks such entries 'prunable' if you want to look first. Prefer git worktree remove [--force] in cleanup code so this never happens.