refactor: migrate trade tools from file-based to SQLite position storage

Complete rewrite of position management in MCP trade tools:

**Trade Tools (agent_tools/tool_trade.py)**
- Replace file-based position.jsonl reads with SQLite queries
- Add get_current_position_from_db() to query positions and holdings tables
- Rewrite buy() and sell() to write directly to database
- Calculate portfolio value and P&L metrics in tools
- Accept job_id and session_id parameters via ContextInjector
- Return errors with proper context for debugging
- Use deployment-aware database path resolution

**Context Injection (agent/context_injector.py)**
- Add job_id and session_id to constructor
- Inject job_id and session_id into buy/sell tool calls
- Support optional parameters (None in standalone mode)

**BaseAgent (agent/base_agent/base_agent.py)**
- Read JOB_ID from runtime config
- Pass job_id to ContextInjector during initialization
- Enable automatic context injection for API mode

**ModelDayExecutor (api/model_day_executor.py)**
- Add _initialize_starting_position() method
- Create initial position record before agent runs
- Load initial_cash from config
- Update context_injector.session_id after session creation
- Link positions to sessions automatically

**Architecture Changes:**
- Eliminates file-based position tracking entirely
- Single source of truth: SQLite database
- Positions automatically linked to trading sessions
- Concurrent execution safe (no file system conflicts)
- Deployment mode aware (prod vs dev databases)

This completes the migration to database-only position storage.
File-based position.jsonl is no longer used or created.

Fixes context injection errors in concurrent simulations.

api/model_day_executor.py

@@ -122,12 +122,20 @@ class ModelDayExecutor:
             session_id = self._create_trading_session(cursor)
             conn.commit()
 
+            # Initialize starting position if this is first day
+            self._initialize_starting_position(cursor, session_id)
+            conn.commit()
+
             # Set environment variable for agent to use isolated config
             os.environ["RUNTIME_ENV_PATH"] = self.runtime_config_path
 
             # Initialize agent
             agent = await self._initialize_agent()
 
+            # Update context injector with session_id
+            if hasattr(agent, 'context_injector') and agent.context_injector:
+                agent.context_injector.session_id = session_id
+
             # Run trading session
             logger.info(f"Running trading session for {self.model_sig} on {self.date}")
             session_result = await agent.run_trading_session(self.date)
@@ -287,6 +295,51 @@ class ModelDayExecutor:
 
         return cursor.lastrowid
 
+    def _initialize_starting_position(self, cursor, session_id: int) -> None:
+        """
+        Initialize starting position if no prior positions exist for this job+model.
+
+        Creates action_id=0 position with initial_cash and zero stock holdings.
+
+        Args:
+            cursor: Database cursor
+            session_id: Trading session ID
+        """
+        # Check if any positions exist for this job+model
+        cursor.execute("""
+            SELECT COUNT(*) FROM positions
+            WHERE job_id = ? AND model = ?
+        """, (self.job_id, self.model_sig))
+
+        if cursor.fetchone()[0] > 0:
+            # Positions already exist, no initialization needed
+            return
+
+        # Load config to get initial_cash
+        import json
+        with open(self.config_path, 'r') as f:
+            config = json.load(f)
+
+        agent_config = config.get("agent_config", {})
+        initial_cash = agent_config.get("initial_cash", 10000.0)
+
+        # Create initial position record
+        from datetime import datetime
+        created_at = datetime.utcnow().isoformat() + "Z"
+
+        cursor.execute("""
+            INSERT INTO positions (
+                job_id, date, model, action_id, action_type,
+                cash, portfolio_value, session_id, created_at
+            )
+            VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)
+        """, (
+            self.job_id, self.date, self.model_sig, 0, "init",
+            initial_cash, initial_cash, session_id, created_at
+        ))
+
+        logger.info(f"Initialized starting position for {self.model_sig} with ${initial_cash}")
+
     async def _store_reasoning_logs(
         self,
         cursor,