move branch-v1 default into checkpointpolicy

After the checkpoint_version metadata field was removed, the exported CheckpointVersionBranchV1 constant no longer described anything in api/checkpoint; its only consumer was checkpointpolicy's defaults, reached through a re-export in the cli checkpoint alias file. Define the constant in checkpointpolicy next to the format vocabulary that already owns it and drop the api const and alias, matching what the CheckpointVersionRefsV1 removal already did.

Also fold the AuthorReader interface into persistent.go (the file holding it had shrunk to that single declaration) and reuse the v1BranchRef test helper in the seam test instead of rebuilding the ref name inline.