feat(corroboration): approval_action decision fn + kill-switch (INV-COR2/COR4, X11 Phase 2)

- HALACHA_CORROBORATION_AUTO_APPROVE config (default ON, Dafna validated 2026-06-01)
- approval_action(agg, has_overruled): overruled→demote, corroborated→approve, else None
- 4 offline unit tests; Phase 2 plan + TaskMaster #75

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/corroboration.py

@@ -52,6 +52,22 @@ def aggregate(links: list[dict], min_cites: int = config.HALACHA_CORROBORATION_M
     }
 
 
+def approval_action(agg: dict, has_overruled: bool) -> str | None:
+    """Decide the corroboration→approval action for ONE halacha (INV-COR2/COR4).
+
+    - 'demote'  : a later court overruled it → back to the chair gate (overruled
+                  outranks any positive count, INV-COR2 strong form).
+    - 'approve' : corroborated (≥N distinct positives, 0 negatives — INV-COR4).
+    - None      : leave as-is (single source, non-overruled negative, or the
+                  uncorroborated tail — INV-COR5 keeps the chair gate).
+    """
+    if has_overruled:
+        return "demote"
+    if agg.get("corroborated"):
+        return "approve"
+    return None
+
+
 _TREATMENT_PROMPT = """אתה משפטן בכיר. נתון ציטוט של פסק/החלטה קודמים בתוך החלטה מאוחרת.
 סווג כיצד ההחלטה המאוחרת **מטפלת** בתקדים המצוטט, לפי אחת מהקטגוריות:
 - followed   — אימצה והחילה את ההלכה.