Add hidden `entire doctor rewrite-checkpoints` test tooling (git-branch → git-refs)

Test tooling for evaluating the git-refs store against real branch data. It re-materializes every checkpoint on the entire/checkpoints/v1 branch as a per-checkpoint ref by re-driving the git-refs store's write path:

• Per session it replays the transcript (regenerating the compact transcript.jsonl), prompts, and metadata through Write, so the tree is rooted at the checkpoint (no shard folders) and SessionFilePaths/CompactTranscriptStart are correct by construction. Per-session summaries and combined attribution are replayed; a tasks/ subtree is grafted in unchanged.
• The checkpoint id is preserved, and each commit keeps the original author — read from the v1-branch commit that wrote the session (metadata carries no author), falling back to the repo git author.

The command is hidden and writes refs locally without pushing (test tooling for now). Idempotent: existing refs are skipped unless --force; --dry-run reports without writing.