fix: inject signature and today_date into trade tool calls for concurrent simulations

Resolves issue where MCP trade tools couldn't access SIGNATURE and TODAY_DATE
during concurrent API simulations, causing "SIGNATURE environment variable is
not set" errors.

Problem:
- MCP services run as separate HTTP processes
- Multiple simulations execute concurrently via ThreadPoolExecutor
- Environment variables from executor process not accessible to MCP services

Solution:
- Add ContextInjector that implements ToolCallInterceptor
- Automatically injects signature and today_date into buy/sell tool calls
- Trade tools accept optional parameters, falling back to config/env
- BaseAgent creates interceptor and updates today_date per session

Changes:
- agent/context_injector.py: New interceptor for context injection
- agent/base_agent/base_agent.py: Create and use ContextInjector
- agent_tools/tool_trade.py: Add optional signature/today_date parameters

Benefits:
- Supports concurrent multi-model simulations
- Maintains backward compatibility with CLI mode
- AI model unaware of injected parameters

agent/context_injector.py

@@ -0,0 +1,50 @@
+"""
+Tool interceptor for injecting runtime context into MCP tool calls.
+
+This interceptor automatically injects `signature` and `today_date` parameters
+into buy/sell tool calls to support concurrent multi-model simulations.
+"""
+
+from typing import Any, Dict
+
+
+class ContextInjector:
+    """
+    Intercepts tool calls to inject runtime context (signature, today_date).
+
+    Usage:
+        interceptor = ContextInjector(signature="gpt-5", today_date="2025-10-01")
+        client = MultiServerMCPClient(config, tool_interceptors=[interceptor])
+    """
+
+    def __init__(self, signature: str, today_date: str):
+        """
+        Initialize context injector.
+
+        Args:
+            signature: Model signature to inject
+            today_date: Trading date to inject
+        """
+        self.signature = signature
+        self.today_date = today_date
+
+    def __call__(self, tool_name: str, tool_input: Dict[str, Any]) -> Dict[str, Any]:
+        """
+        Intercept tool call and inject context parameters.
+
+        Args:
+            tool_name: Name of the tool being called
+            tool_input: Original tool input parameters
+
+        Returns:
+            Modified tool input with injected context
+        """
+        # Only inject for trade tools (buy/sell)
+        if tool_name in ["buy", "sell"]:
+            # Inject signature and today_date if not already provided
+            if "signature" not in tool_input:
+                tool_input["signature"] = self.signature
+            if "today_date" not in tool_input:
+                tool_input["today_date"] = self.today_date
+
+        return tool_input