review: strip markdown code-fences from JSON verdicts

Haiku 4.5 occasionally wraps its JSON response in a markdown code
fence even with response_format={"type":"json_object"} enforced:

    ```json
    {"clean": true, "reason": "polished read"}
    ```

Live testing the new reviewer caught this — every verdict was being
dropped as "reviewer returned non-JSON". Strip a single leading
trailing fence before json.loads. Defensive for any model that does
the same (Claude variants commonly fence JSON even when told not to).

Adds a unit test covering fenced output.

tests/test_output_review.py

@@ -109,6 +109,25 @@ async def test_review_unclean_verdict(monkeypatch):
     assert "chain of thought" in v.reason
 
 
+@pytest.mark.asyncio
+async def test_review_strips_markdown_fence_around_json(monkeypatch):
+    """Haiku (and friends) sometimes wrap JSON in ```json ... ``` even
+    when response_format is set. The parser needs to peel that off
+    before json.loads or it'll reject otherwise-valid verdicts."""
+    _configure(monkeypatch)
+    fenced = '```json\n{"clean": true, "reason": "polished read"}\n```'
+    def handler(_req):
+        return httpx.Response(200, json={
+            "choices": [{"message": {"content": fenced},
+                         "finish_reason": "stop"}],
+            "usage": {"prompt_tokens": 50, "completion_tokens": 18, "cost": 0.0006},
+        })
+    async with httpx.AsyncClient(transport=_mock_post(handler)) as client:
+        v = await review_read(client, "Markets are pricing tighter policy.")
+    assert v.clean is True
+    assert v.reason == "polished read"
+
+
 @pytest.mark.asyncio
 async def test_review_failsafe_on_malformed_json(monkeypatch):
     """Reviewer returned prose instead of JSON → conservative reject."""