fix(checkpoint-remote): withhold checkpoints when the store has no URL

A checkpoint_remote that passes the ownership vote but whose provider yields no URL (no known host, e.g. "entire" with a github.com origin) fell back to the push remote, so pre-push delivered session checkpoints to the code repository the user had explicitly routed them away from.

PushURL now returns ErrCheckpointRemoteUnroutable in that case. Pre-push skips the checkpoint push, keeps the refs queued and says so on stderr; PushQueuedCheckpointRefs returns the error. Inherited (refused) stores and self-hosted origins are unchanged.