test: add real-hook push helpers for checkpoint sync (I-2)

The real git-invoked pre-push hook was never asserted end to end. Integration RunPrePush spawned the hook with Stdin=nil, and GitPush always passed --no-verify, so a completely broken hook would go unnoticed; e2e remote tests masked the same gap by always calling PushCheckpointRefs explicitly.

Integration TestEnv gains:

• InstallRealPrePushHook + GitPushWithHooks: a plain git push (no --no-verify) that runs the installed hook exactly as git runs it — realistic stdin refspec lines and remote name/URL argv, inheriting the checkpoint-store env.
• RunPrePush now feeds realistic stdin (defaultPrePushStdin) instead of nil, with RunPrePushWithStdin as an escape hatch for the empty-stdin no-op case. GitPush keeps --no-verify for setup plumbing.

A smoke test (seed of A1) proves GitPushWithHooks lands checkpoints on a bare remote with NO explicit checkpoint push, under both backends via ForEachBackend.

e2e gains AssertCheckpointsOnRemote(t, s, bareDir), asserting the backend-appropriate refs are present on the bare remote for later use by the section-3 e2e tests.