Add git-refs per-checkpoint checkpoint backend

Second slice of #1471: a git-backed store that keeps one commit per refs/entire/checkpoints/<shard>/<id> (tree root = checkpoint contents), sharing the treeWriter subtree core with the git-branch store.

• refs_store.go: gitRefsStore implements PersistentStore over per-checkpoint refs (orphan-then-parented history, stamps refs-v1, no Vercel merge, List enumerates local refs, optional AuthorReader). Reads resolve a ref → commit tree and use shared read helpers; writes build the subtree via the embedded *treeWriter.
• Share the tree-read helpers: the git-branch reader's Read/ReadSession* now delegate to readSummaryFromCheckpointTree / readSession*FromTree free functions (no behavior change) so both backends read the same way, just navigating to the tree differently.
• registry: BackendTypeGitRefs ("git-refs") registered built-in with gitBacked:true
  • gitRefsBackendFactory; OpenEnv/OpenOptions gain RefFetcher; PrimaryIsRefs helper.
• pushqueue.go: flock-protected JSONL push-discovery queue in the git common dir (Enqueue/Drain/Remove); gitRefsStore.setRef enqueues best-effort. (Pre-push consumption lands in the next commit.)
• CheckpointVersionRefsV1 = "refs-v1".