`doctor` helps user make deployment preflight CI-ready

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User story

As a dev maintaining CI pipelines, I want to run deployment-relevant doctor checks in machine-readable form, so I can fail early before packaging or deploying.

Acceptance criteria

doctor --format json supports selecting the deploy-preflight check category and returns stable exit semantics.

Readiness checklist

• Acceptance criteria are clearly defined.
• Backward compatibility impact addressed.
• Documentation written.
• Tests added or adjusted accordingly.