Add optional Markdown or SARIF output for CI review

[LarryLemonBot]

Roadmap item for maintainer automation.

Goal:
Add an optional output mode that lets maintainers attach GateCheck findings to pull requests, issues, or release checks.

Acceptance criteria:

• Default CLI behavior remains JSON.
• Markdown/SARIF output is generated from the same observed evidence model.
• Output does not include response bodies, credentials, private URLs, raw payment headers, or unsupported endorsement/security claims.