[PATCH] refs/files: avoid packed-refs lock for root ref deletion

Hi,

Deleting root refs in the files backend unnecessarily locks
packed-refs, even though root refs cannot be packed. This can cause
post-commit cleanup to report an error after a successful commit in a
linked worktree with read-only shared metadata.

The attached patch skips that lock for root-ref deletion and adds
regression tests. All seven new tests fail without the fix and pass
with it; broader ref, worktree, and sequencer tests also pass.

AI assistance was used to generate the patch, tests, and commit message.

Thanks,
Ariel