feat: cross-verification protocol

[4444J99]

CrossVerifier checks output against handoff constraints: locked files, never-touch patterns, convention drift. PR #10. DONE-285.

[github-actions]

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[4444J99]

Engagement disposition: distilled:conductor/cross_verify.py. I verified CrossVerifier is present on main and checks changed files against GuardrailedHandoffBrief constraints: locked files, receiver never-touch patterns, and convention drift warnings. mcp_server.py exposes conductor_fleet_cross_verify and clears the active handoff when verification passes, with tests in tests/test_cross_verify.py.