Add primary onboarding commands and workflow help

[stefanvictora]

What to build

Make the common setup path easier to discover by promoting user-intent commands and showing the typical revpack workflow directly in CLI help.

Acceptance criteria

• Add top-level revpack connect with the same behavior as revpack config setup.
• Add top-level revpack doctor with the same behavior and options as revpack config doctor, including --profile and --json.
• Keep revpack config setup and revpack config doctor working for compatibility, but stop actively promoting them in next-step guidance and primary workflow docs.
• Add revpack setup --agent <target> to create REVIEW.md if missing and install the selected agent adapter.
• Keep revpack setup as review-guidance setup only.
• Keep revpack setup agent <target> as adapter-only setup.
• Add a concise top-level help workflow that covers connect, doctor, setup --agent, prepare, agent run, status, publish, checkout for another PR/MR, and local branch review.
• Add brief checkout --help examples for common target forms without turning help into a full command reference.
• Update README/docs, CLI help export tests, command tests, and changelog for the new primary onboarding path.

Blocked by

None - can start immediately