Test entire:// mirror checkpoint routing via resolvePushSettings

Add two tests exercising the production push path (resolvePushSettings -> remote.PushURL -> deriveCheckpointURLFromInfo) for entire:// push-through mirrors:

• a forge-matching mirror routes checkpoints through the same-cluster mirror (entire://<cluster>/gh/<repo>) instead of a direct provider URL
• a forge-mismatched mirror falls back to the provider's canonical host

Assertions use a real repo and real entire:// remote and check the derived target. entire:// can't be pushed for real without the git-remote-entire helper and a live cluster, so this is the closest end-to-end coverage feasible without infra.