Address PR feedback: guaranteed enqueue, cleanup parity

MigrateBranchToRefs promised refs are queued for push, but setRef's enqueue is best-effort — a failed enqueue left migrated refs written locally and silently never pushed. The migration now enqueues explicitly and fails on error; duplicates collapse on Drain.

PushQueuedCheckpointRefs now runs post-push shadow-branch cleanup after a successful flush, matching the pre-push paths.