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/tests/test_corroboration.py

@@ -44,3 +44,24 @@ def test_aggregate_negative_blocks():
 def test_aggregate_below_threshold():
     agg = cor.aggregate([_link("d1","followed")], min_cites=2)
     assert agg["corroborated"] is False           # single source insufficient (INV-COR4)
+
+
+# --- Phase 2: approval decision (INV-COR2/COR4) ---
+
+def test_approval_action_corroborated_approves():
+    agg = {"positive_sources": 2, "has_negative": False, "corroborated": True}
+    assert cor.approval_action(agg, has_overruled=False) == "approve"
+
+def test_approval_action_overruled_demotes_even_if_corroborated():
+    # overruled outranks any positive count (INV-COR2 strong form)
+    agg = {"positive_sources": 3, "has_negative": True, "corroborated": False}
+    assert cor.approval_action(agg, has_overruled=True) == "demote"
+
+def test_approval_action_single_source_noop():
+    agg = {"positive_sources": 1, "has_negative": False, "corroborated": False}
+    assert cor.approval_action(agg, has_overruled=False) is None
+
+def test_approval_action_negative_nonoverruled_noop():
+    # distinguished blocks approval but does not demote (no overruled)
+    agg = {"positive_sources": 2, "has_negative": True, "corroborated": False}
+    assert cor.approval_action(agg, has_overruled=False) is None