Polish agent checkpoints (#29265)

Release Notes:

- Improved performance of agent checkpoint creation.
- Fixed a bug that sometimes caused accidental deletions when restoring
to a previous agent checkpoint.
- Fixed a bug that caused checkpoints to be visible in the Git history.

crates/fs/src/fake_git_repo.rs

@@ -431,10 +431,6 @@ impl GitRepository for FakeGitRepository {
         unimplemented!()
     }
 
-    fn delete_checkpoint(&self, _checkpoint: GitRepositoryCheckpoint) -> BoxFuture<Result<()>> {
-        unimplemented!()
-    }
-
     fn diff_checkpoints(
         &self,
         _base_checkpoint: GitRepositoryCheckpoint,