fix(checkpoint-remote): report withheld checkpoints in attach, status, topology

Follow-up to the unroutable-store withhold: three callers still read an unroutable checkpoint_remote as "no dedicated store" and described the push remote as the destination.

• CheckpointPushTarget returns the withhold error, so attach no longer confirms delivery against the push remote (which may hold a stale copy) and reports "Pushed checkpoint metadata" for a checkpoint that went nowhere.
• status reports "Checkpoints NOT syncing" instead of naming the elected remote.
• remote topology treats the remote as pinned, since pre-push bypasses the sync-remote election for it.