M2: Add CI parity reporting for local blocking checks

[dbrosio3]

Goal

Report whether every local blocking check has a CI mirror or an explicit local-only exception.

User value

Teams can trust that local blockers are convenience feedback, while CI remains the final enforceable gate.

Implementation notes

• Add config validation for ci_mirror or equivalent fields.
• Warn or fail when a blocking pre-push check lacks a CI mirror and no exception is declared.
• Include a summary in pushgate doctor or equivalent diagnostics.
• Keep warnings understandable for teams still adopting CI mirrors gradually.

Risks / tradeoffs

• Strict parity may frustrate teams with checks that cannot run in CI.
• Too much flexibility may weaken the product contract.

Suggested priority

P1

Milestone

M2: CI mirror

TBD considerations

• Whether CI parity violations should warn or fail by default.
• How to model valid local-only exceptions.

Acceptance criteria

• Config validation identifies unmirrored blocking checks.
• Local-only exceptions are explicit and documented.
• Output links users to CI mirror guidance.