When archiving a thread's linked worktree, `git worktree remove` can
fail to delete the working directory while still removing the admin
metadata in `.git/worktrees/<name>`. This leaves an orphaned folder on
disk that no longer appears in `git worktree list`.

## Fix

Delete the directory ourselves first with `std::fs::remove_dir_all`,
then call `git worktree remove --force` to clean up only the admin
entry. Git already handles the case where the working tree is already
gone (since
[git/git@ee6763a](https://github.com/git/git/commit/ee6763af0a3b97225803c6c908a29de40336cf38)).

Also fixes `FakeGitRepository::remove_worktree` to handle a missing
working tree directory (matching real git behavior) by scanning
`.git/worktrees/` entries when the `.git` pointer file can't be read.

Release Notes:

- Fixed archiving an agent thread sometimes leaving the worktree folder
on disk even though the git worktree was removed.