docs: add development mode documentation

Add comprehensive development mode documentation to README.md, API_REFERENCE.md, and CLAUDE.md:

README.md:
- New "Development Mode" section after Configuration
- Quick start guide with environment variables
- Explanation of DEV vs PROD mode behavior
- Mock AI behavior and stock rotation details
- Environment variables reference
- Use cases and limitations

API_REFERENCE.md:
- New "Deployment Mode" section after health check
- Response format with deployment_mode fields
- DEV mode behavior explanation
- Health check example with deployment fields
- Use cases for testing and CI/CD

CLAUDE.md:
- New "Development Mode" subsection in Important Implementation Details
- Deployment modes overview
- DEV mode characteristics
- Implementation details with file references
- Testing commands and mock behavior notes

All sections explain:
- DEPLOYMENT_MODE environment variable (PROD/DEV)
- PRESERVE_DEV_DATA flag for dev data persistence
- Mock AI provider with deterministic stock rotation
- Separate dev database and data paths
- Use cases for development and testing

CLAUDE.md

@@ -294,6 +294,37 @@ bash main.sh
 - Logs include timestamps, signature, and all message exchanges
 - Position updates append to single `position/position.jsonl`
 
+**Development Mode:**
+
+AI-Trader supports a development mode that mocks AI API calls for testing without costs.
+
+**Deployment Modes:**
+- `DEPLOYMENT_MODE=PROD`: Real AI calls, production data paths
+- `DEPLOYMENT_MODE=DEV`: Mock AI, isolated dev environment
+
+**DEV Mode Characteristics:**
+- Uses `MockChatModel` from `agent/mock_provider/`
+- Data paths: `data/dev_agent_data/` and `data/trading_dev.db`
+- Dev database reset on startup (controlled by `PRESERVE_DEV_DATA`)
+- API responses flagged with `deployment_mode` field
+
+**Implementation Details:**
+- Deployment config: `tools/deployment_config.py`
+- Mock provider: `agent/mock_provider/mock_ai_provider.py`
+- LangChain wrapper: `agent/mock_provider/mock_langchain_model.py`
+- BaseAgent integration: `agent/base_agent/base_agent.py:146-189`
+- Database handling: `api/database.py` (automatic path resolution)
+
+**Testing Dev Mode:**
+```bash
+DEPLOYMENT_MODE=DEV python main.py configs/default_config.json
+```
+
+**Mock AI Behavior:**
+- Deterministic stock rotation (AAPL → MSFT → GOOGL → etc.)
+- Each response includes price query, buy order, and finish signal
+- No actual AI API calls or costs
+
 ## Testing Changes
 
 When modifying agent behavior or adding tools: