test(e2e): rely on the real pre-push hook for remote clone tests (G1)

The remote clone tests pushed checkpoints explicitly via PushCheckpointRefs right after the user push, which would mask a broken pre-push hook. Replace those calls with AssertCheckpointsOnRemote so a plain git push must sync checkpoints through the installed hook end to end (A5/G1 in spirit).

No prompt text changed, so the vogon canary parser is unaffected. Verified green under both E2E_CHECKPOINT_STORE values.