Add `remove_worktree()` and `rename_worktree()` to the `GitRepository`
trait with `RealGitRepository` implementations that shell out to `git
worktree remove/move`.
Implement all 4 worktree methods (`worktrees`, `create_worktree`,
`remove_worktree`, `rename_worktree`) on `FakeGitRepository` backed by
`FakeGitRepositoryState`, with `simulated_create_worktree_error` for
test-time fault injection.
Add `set_create_worktree_error()` helper on `FakeFs`.
Add `parse_worktrees_from_str` helper and 7 new tests covering real git
operations and fake worktree lifecycle.
Closes AI-31
Release Notes:
- N/A
Richard Feldman
f59baa942a