feat: post-merge moot validation wording + validation:* labels (#529)

Adds #529 acceptance criteria 1/4/5.

- post_merge_validation.py: assess_post_merge_validation defines the four
  canonical validation distinctions (clean pass / baseline-accepted / blocked /
  post-merge moot) and fails closed on two mistakes: (a) an active approval on a
  PR already merged/closed before review submission, and (b) post-merge moot
  validation claimed without merged-state + merge-commit proof.
- issue_workflow_labels.py: durable validation:* process-state labels
  (clean-pass / baseline-accepted / blocked / post-merge-moot) registered in
  CANONICAL_LABEL_SPECS so manage_labels creates them.
- final_report_validator.py: _rule_reviewer_post_merge_validation wired into
  the review_pr rule set.
- tests/test_post_merge_validation.py: module behavior, label registration,
  and the wired review rule.

Refs #529

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

final_report_validator.py

@@ -855,6 +855,27 @@ def _rule_reviewer_premerge_baseline_proof(report_text: str) -> list[dict[str, s
     )
 
 
+def _rule_reviewer_post_merge_validation(report_text: str) -> list[dict[str, str]]:
+    """Block active approval on an already-merged/closed PR, and post-merge moot
+    validation claimed without merged-state + merge-commit proof (#529)."""
+    from post_merge_validation import assess_post_merge_validation
+
+    result = assess_post_merge_validation(report_text)
+    if not result.get("block"):
+        return []
+    return _findings_from_reasons(
+        "reviewer.post_merge_validation",
+        result.get("reasons") or [],
+        field="Validation status",
+        severity="block",
+        safe_next_action=result.get("safe_next_action")
+        or (
+            "record post-merge moot validation instead of an active approval; "
+            "cite PR state merged/closed and the merge commit SHA"
+        ),
+    )
+
+
 def _rule_reviewer_validation_status_vocabulary(
     report_text: str,
     *,
@@ -1519,6 +1540,7 @@ _RULES_BY_TASK: dict[str, list[Callable[..., list[dict[str, str]]]]] = {
         _rule_reviewer_linked_issue,
         _rule_reviewer_baseline_on_failure,
         _rule_reviewer_premerge_baseline_proof,
+        _rule_reviewer_post_merge_validation,
         _rule_reviewer_validation_status_vocabulary,
         _rule_reviewer_main_checkout_baseline,
         _rule_reviewer_main_checkout_path,