feat(#496): fail-closed canonical comment validation before Gitea posts

Add canonical_comment_validator and wire it into issue comments, PR review
bodies, reconcile post_comment, and structured comment helpers. Workflow-changing
comments without complete next-action state are rejected before any API call.
Includes unit/MCP integration tests, runbook docs, and a final-report rule
blocking false "comment posted" claims when validation failed.

gitea_mcp_server.py

@@ -829,6 +829,7 @@ import review_merge_state_machine  # noqa: E402
 import pr_work_lease  # noqa: E402
 import native_mcp_preference  # noqa: E402
 import worktree_cleanup_audit  # noqa: E402
+import canonical_comment_validator as ccv  # noqa: E402
 
 
 # Keyed issue-lock storage (#443): per remote/org/repo/issue files under
@@ -3268,6 +3269,13 @@ def _evaluate_pr_review_submission(
         result["pr_work_lease"] = lease_block
         return result
 
+    if (body or "").strip():
+        gate = _canonical_comment_gate(body, context="pr_review")
+        if gate["blocked"]:
+            reasons.extend(gate["reasons"])
+            result["canonical_comment_validation"] = gate["canonical_comment_validation"]
+            return result
+
     result["would_perform"] = True
     if not live:
         reasons.append(
@@ -5207,6 +5215,12 @@ def gitea_reconcile_already_landed_pr(
                 "gitea.pr.comment"
             )
             return result
+        gate = _canonical_comment_gate(comment_body)
+        if gate["blocked"]:
+            result["success"] = False
+            result["reasons"].extend(gate["reasons"])
+            result["canonical_comment_validation"] = gate["canonical_comment_validation"]
+            return result
         comment_url = f"{base}/issues/{pr_number}/comments"
         with _audited(
             "comment_pr",
@@ -6444,6 +6458,15 @@ def gitea_create_issue_comment(
             blocked["permission_report"] = _permission_block_report(
                 "gitea.issue.comment")
         return blocked
+    canon_gate = _canonical_comment_gate(body, context="issue_comment")
+    if canon_gate["blocked"]:
+        return {
+            "success": False,
+            "performed": False,
+            "issue_number": issue_number,
+            "reasons": canon_gate["reasons"],
+            "canonical_comment_validation": canon_gate["canonical_comment_validation"],
+        }
     h, o, r = _resolve(remote, host, org, repo)
     auth = _auth(h)
     api = f"{repo_api_url(h, o, r)}/issues/{issue_number}/comments"
@@ -7973,6 +7996,34 @@ def gitea_audit_config() -> dict:
     return report
 
 
+def _canonical_comment_gate(
+    body: str,
+    *,
+    context: str | None = None,
+    force_workflow: bool = False,
+) -> dict:
+    """Fail-closed canonical state validation before comment/review POST (#496)."""
+    assessment = ccv.assess_canonical_comment(
+        body,
+        context=context,
+        force_workflow=force_workflow,
+    )
+    if assessment.get("allowed"):
+        return {
+            "blocked": False,
+            "canonical_comment_validation": assessment,
+            "reasons": [],
+        }
+    correction = (assessment.get("correction_message") or "").strip()
+    return {
+        "blocked": True,
+        "canonical_comment_validation": assessment,
+        "reasons": [
+            correction or "canonical comment validation failed (fail closed before posting)"
+        ],
+    }
+
+
 def _post_structured_issue_comment(
     *,
     issue_number: int,
@@ -7982,8 +8033,18 @@ def _post_structured_issue_comment(
     org: str | None,
     repo: str | None,
     audit_op: str = "create_issue_comment",
+    comment_context: str | None = None,
 ) -> dict:
     """Post an issue-thread comment after permission gates already passed."""
+    gate = _canonical_comment_gate(body, context=comment_context)
+    if gate["blocked"]:
+        return {
+            "success": False,
+            "performed": False,
+            "issue_number": issue_number,
+            "reasons": gate["reasons"],
+            "canonical_comment_validation": gate["canonical_comment_validation"],
+        }
     h, o, r = _resolve(remote, host, org, repo)
     auth = _auth(h)
     api = f"{repo_api_url(h, o, r)}/issues/{issue_number}/comments"