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/graph/trading_graph.py

@@ -40,7 +40,8 @@ from tradingagents.agents.utils.agent_utils import (
     get_news,
     get_insider_transactions,
     get_global_news,
-    get_macro_indicators
+    get_macro_indicators,
+    get_prediction_markets
 )
 
 from .checkpointer import checkpoint_step, clear_checkpoint, get_checkpointer, thread_id
@@ -194,6 +195,7 @@ class TradingAgentsGraph:
                     get_global_news,
                     get_insider_transactions,
                     get_macro_indicators,
+                    get_prediction_markets,
                 ]
             ),
             "fundamentals": ToolNode(