Worktree cleanup warning now gives submodule folders as its example

A worktree is an extra working copy of a git repository, kept in its own folder. When Claude Code drops its record of a worktree, it shows a warning that says what the folder held. That warning used to give "a reftable repository's refs" as its example. It now says the folder held "a submodule's git directory" instead. A submodule is one git repository nested inside another.