codeSplitter: Dependency doctor: verify updates with a preview deploy, then tear down #864

Part of #859.

Check updates against a real deploy, not just the tests: once the doctor's branch passes, provision a preview, deploy it, run the check, then tear it down.

• Reuses the existing preview deploys and checks (CiRunner / DeployRunner), so the repo needs no CI setup of its own
• A failed deploy or check feeds the on-fail chain like a test failure would
• Respects the existing limits: container minutes, preview-deploy rate limit, and "only a person ships" to prod (this is preview only)
• An opt-in per repo, since it costs deploy time