Add safe AI corpus propagation controls

[woksin]

Summary

Adds fail-closed controls for pausing and deliberately restoring AI corpus distribution while the organization moves away from all-to-all propagation.

Added

• Add snapshot-backed freeze, verification, restore, canary, and single-source controls for AI corpus propagation (Replace all-to-all AI corpus propagation with a controlled distribution model #68)
• Add a protected manual control workflow and mocked offline safety tests (Replace all-to-all AI corpus propagation with a controlled distribution model #68)

Changed

• Harden GitHub API retries for rate limits and transient failures while preventing duplicate non-idempotent dispatches (Replace all-to-all AI corpus propagation with a controlled distribution model #68)
• Document operator safeguards and the current freeze's restore limitations (Replace all-to-all AI corpus propagation with a controlled distribution model #68)