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]>

issue_workflow_labels.py

@@ -42,12 +42,38 @@ STATUS_LABEL_SPECS: tuple[LabelSpec, ...] = (
     LabelSpec("status:wontfix", "000000", "Issue will not be fixed"),
 )
 
+# Durable validation-outcome labels (#529): the four canonical distinctions a
+# reviewer report can carry. These are orthogonal to the single active status:*
+# label, so they use their own prefix and are not subject to the one-status
+# invariant.
+VALIDATION_LABEL_SPECS: tuple[LabelSpec, ...] = (
+    LabelSpec("validation:clean-pass", "0e8a16", "Validation was a clean pass"),
+    LabelSpec(
+        "validation:baseline-accepted",
+        "fbca04",
+        "Validation passed with a baseline-proven unrelated failure",
+    ),
+    LabelSpec(
+        "validation:blocked",
+        "b60205",
+        "Validation blocked by an unresolved failure",
+    ),
+    LabelSpec(
+        "validation:post-merge-moot",
+        "5319e7",
+        "Validation is post-merge moot (PR already merged/closed before review)",
+    ),
+)
+
 CANONICAL_LABEL_SPECS: tuple[LabelSpec, ...] = (
-    TYPE_LABEL_SPECS + STATUS_LABEL_SPECS
+    TYPE_LABEL_SPECS + STATUS_LABEL_SPECS + VALIDATION_LABEL_SPECS
 )
 
 TYPE_LABELS: frozenset[str] = frozenset(spec.name for spec in TYPE_LABEL_SPECS)
 STATUS_LABELS: frozenset[str] = frozenset(spec.name for spec in STATUS_LABEL_SPECS)
+VALIDATION_LABELS: frozenset[str] = frozenset(
+    spec.name for spec in VALIDATION_LABEL_SPECS
+)
 CANONICAL_LABELS: frozenset[str] = frozenset(
     spec.name for spec in CANONICAL_LABEL_SPECS
 )