Worktrees in /tmp or other ephemeral locations get cleaned by the OS
but remain in git's worktree list as "prunable". These stale entries
caused branches to show a worktree label pointing at a non-existent
directory, leading to broken state when selected.

Now validates each worktree path with existsSync before including it
in the branch→worktreePath map.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>