feat: add unsupported claim governance finding

[CoderDeltaLAN]

Summary

Adds the first v0.2 governance finding: AIRK-GOV006, a conservative local diagnostic for unsupported security or maturity claims inside supported agent instruction files.

Scope

• Adds agent_rules_kit.governance.
• Reports findings in console, JSON, and Markdown check output.
• Adds fixture coverage for unsupported claims.
• Adds false-positive tests for negative guidance such as "do not claim" and "not a security scanner".
• Keeps runtime local-only: no network, no LLM, and no repository command execution.

Verification

• ./scripts/check.sh
• console smoke for tests/fixtures/repositories/unsupported-claim
• JSON smoke for tests/fixtures/repositories/unsupported-claim
• Markdown smoke for tests/fixtures/repositories/unsupported-claim
• clean fixture JSON smoke with finding_count: 0
• isolated build + wheel install smoke

Product boundary

This does not turn agent-rules-kit into a security scanner. It flags unsupported claims conservatively so instruction files stay professional, auditable, and honest.