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/default_config.py

@@ -107,6 +107,7 @@ DEFAULT_CONFIG = _apply_env_overrides({
         "fundamental_data": "yfinance",      # Options: alpha_vantage, yfinance
         "news_data": "yfinance",             # Options: alpha_vantage, yfinance
         "macro_data": "fred",                # Options: fred (needs FRED_API_KEY)
+        "prediction_markets": "polymarket",  # Options: polymarket (keyless)
     },
     # Tool-level configuration (takes precedence over category-level)
     "tool_vendors": {