reject unnumbered trails and stale worktrees

Worktree checkout now requires a trail number: sanitized branch names are lossy, so the number is the directory's only disambiguator, and sibling commands already reject unnumbered trails.

A branch registered to a hand-deleted worktree directory now fails with a 'git worktree prune' hint instead of pointing the cd hint at a missing path; pruning stays the user's call.