docs: index governance class taxonomy

Author: LalaSkye

docs/governance/README.md

@@ -10,6 +10,7 @@ Each file has one job.
 | `INVARIANTS.md` | What must remain true. |
 | `BUILD_RECEIPTS.md` | What has changed and what claim boundary is held. |
 | `CHANGE_CONTROL.md` | When a change requires a receipt or claim-boundary review. |
+| `GOVERNANCE_CLASSES.md` | Which governance trigger classes are permitted. |
 | `VERIFICATION_RECEIPT_SCOPE.md` | What local verification receipts are allowed to prove. |
 | `CI_TRIGGER_RECEIPT.md` | Why the first CI trigger commit exists and what it does not prove. |