feat(data): add Polymarket prediction markets as a keyless vendor

Surface live, market-implied probabilities for forward-looking events (Fed
decisions, recession, elections, geopolitics, crypto) to the news analyst via a
new get_prediction_markets tool and a prediction_markets vendor category. Backed
by Polymarket's public Gamma API (no key). Results are filtered to open,
forward-looking markets (closed and past-dated events excluded), ranked by
traded volume, and rendered with implied probability, volume, resolution date,
and the recent move. External errors degrade to a clear unavailable message
rather than interrupting the analyst.

tradingagents/agents/utils/agent_utils.py

@@ -26,6 +26,9 @@ from tradingagents.agents.utils.news_data_tools import (
 from tradingagents.agents.utils.macro_data_tools import (
     get_macro_indicators
 )
+from tradingagents.agents.utils.prediction_markets_tools import (
+    get_prediction_markets
+)
 from tradingagents.agents.utils.market_data_validation_tools import (
     get_verified_market_snapshot
 )