debug: enable stdout/stderr for MCP services to diagnose parameter injection

MCP services were started with stdout/stderr redirected to DEVNULL, making
debug logs invisible. This prevented diagnosing why _current_position parameter
is not being received by buy() function.

Changed subprocess.Popen to redirect MCP service output to main process
stdout/stderr, allowing [DEBUG buy] logs to be visible in docker logs.

This will help identify whether:
1. _current_position is being sent by ContextInjector but not received
2. MCP HTTP transport filters underscore-prefixed parameters
3. Parameter serialization is failing

Related to negative cash bug where final position shows -$3,049.83 instead
of +$727.92 tracked by ContextInjector.

agent_tools/start_mcp_services.py

@@ -78,10 +78,11 @@ class MCPServiceManager:
             env['PYTHONPATH'] = str(Path.cwd())
 
             # Start service process (output goes to Docker logs)
+            # Enable stdout/stderr for debugging (previously sent to DEVNULL)
             process = subprocess.Popen(
                 [sys.executable, str(script_path)],
-                stdout=subprocess.DEVNULL,
+                stdout=sys.stdout,  # Redirect to main process stdout
-                stderr=subprocess.DEVNULL,
+                stderr=sys.stderr,  # Redirect to main process stderr
                 cwd=Path.cwd(),  # Use current working directory (/app)
                 env=env  # Pass environment with PYTHONPATH
             )