fix: resolve KeyError 'data_dir' when running trade execution

PortfolioRepository expects its own portfolio config (with data_dir),
not DEFAULT_CONFIG. Passing config=self.config caused a KeyError because
DEFAULT_CONFIG has no data_dir key. Removing the argument lets the
repository call get_portfolio_config() which provides the correct defaults.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent_os/backend/services/langgraph_engine.py

@@ -390,7 +390,7 @@ class LangGraphEngine:
         _store = store or ReportStore()
 
         try:
-            repo = PortfolioRepository(config=self.config)
+            repo = PortfolioRepository()
             executor = TradeExecutor(repo=repo, config=self.config)
 
             # Execute decisions