fix(MAT-174): Robust LLM filter JSON extraction with regex

LLM sometimes returns extra text around the JSON array. Use regex to
extract the array pattern instead of parsing the full response.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cron/brave_search.py

@@ -59,8 +59,13 @@ async def _llm_filter(results: list[dict], criteria: str) -> list[dict]:
             data = resp.json()
 
         reply = data["choices"][0]["message"]["content"].strip()
-        # Parse the JSON array of indices
-        indices = json.loads(reply)
+        # Extract JSON array from response (LLM may include extra text)
+        import re
+        match = re.search(r"\[[\d,\s]*\]", reply)
+        if not match:
+            logger.warning("LLM filter returned no array: %s", reply)
+            return results
+        indices = json.loads(match.group())
         if not isinstance(indices, list):
             logger.warning("LLM filter returned non-list: %s", reply)
             return results