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.

agent/base_agent/base_agent.py

@@ -173,10 +173,15 @@ class BaseAgent:
                 print("⚠️  OpenAI base URL not set, using default")
 
         try:
+            # Get job_id from runtime config if available (API mode)
+            from tools.general_tools import get_config_value
+            job_id = get_config_value("JOB_ID")  # Returns None if not in API mode
+
             # Create context injector for injecting signature and today_date into tool calls
             self.context_injector = ContextInjector(
                 signature=self.signature,
-                today_date=self.init_date  # Will be updated per trading session
+                today_date=self.init_date,  # Will be updated per trading session
+                job_id=job_id  # Will be None in standalone mode, populated in API mode
             )
 
             # Create MCP client with interceptor