/simplify: reuse refBase, drop dead branch, rename push entry

Resolve the existing ref through the refs store's refBase instead of an inline Reference/CommitObject chain, keeping the same semantics: a ref that doesn't resolve to a readable commit is treated as absent and the checkpoint is re-migrated as an orphan (pinned by a new test).

Drop the unreachable malformed-id skip — WalkCheckpointShards only yields validated IDs, and refBase would surface the error anyway.

Rename PushMigratedCheckpointRefs to PushQueuedCheckpointRefs: it flushes the whole queue, not just migrated refs.