feat(opf): show held checkpoints in entire status and suggest git-refs

entire status (text and --json checkpoint_opf_pending) reports how many checkpoints are held until the background scan covers them, on both backends: queued refs without the trailer on git-refs, unpushed untrailered v1 commits on git-branch. The count reads local refs only and is omitted when OPF is off.

When v1 is held for a scan, git-branch users see a tip, at most once every 30 days per repository, pointing at entire doctor migrate-checkpoints.