Add governance approval workflow

[NavinAgrawal]

Summary

• add local expiring governance approval records for rollout, rollback, policy override, and break-glass requests
• add mcp-broker governance approve and make governance-approve
• document the approval workflow and phase roadmap status

Verification

• Private: make precommit PYTEST_WORKERS=0 -> 1357 unit passed, 150 journey passed, 0 failed, 0 skipped
• Private: make maintainer-violations PYTEST_WORKERS=0 -> 0 violations, 0 critical/high/medium/low, 12 test-file-size advisories
• Private: make maintainer-grade-quality PYTEST_WORKERS=0 -> A+ 100.0
• Public: make precommit PYTEST_WORKERS=0 -> 1357 unit passed, 116 journey passed, 0 failed, 0 skipped
• Public: make maintainer-violations PYTEST_WORKERS=0 -> 0 violations, 0 critical/high/medium/low, 12 test-file-size advisories
• Public: make maintainer-grade-quality PYTEST_WORKERS=0 -> A+ 100.0
• make public-export-check PUBLIC_REPO=/Users/nba/Projects/public/mcp-broker PYTEST_WORKERS=0

Security

• GitHub secret scanning is disabled on this repo
• GitGuardian HEAD check-runs are not failing