docs: add git-refs condensation/push scenario to checkpoint-scenarios

The scenarios doc covered only the git-branch condense-to-v1 flow. Add Scenario 8 showing how the git-refs backend differs — condensing each checkpoint to refs/entire/checkpoints/<shard>/<id> and pushing via the push-discovery queue (fast-forward-only, with fetch+replay recovery) — while stressing that all session/timing/overlap logic from scenarios 1–7 is backend-orthogonal and unchanged. Add a matching summary-table row and a pointer to the ref-backend doc.