Fetch missing git-refs checkpoints on demand for reads

A git-refs primary previously read only checkpoint refs already present locally, so checkpoints written on another machine (or not yet fetched after clone) read as not-found. FetchCheckpointRef fetches one ref via the checkpoint remote into the local ref of the same name; it's wired into the read commands that already inject FetchBlobsByHash (resume, explain, attribution, tokens profile) via the new OpenOptions.RefFetcher. gitRefsStore.resolveRefMaybeFetch invokes it once on a local miss and retries; a failed fetch resolves to ErrCheckpointNotFound. The git-branch backend ignores it; strategy-side Opens (condensation/rewind) read just-written local refs and pass no fetcher.