B1: LLM-Powered Chat Engine

## Completed (2026-03-08)
Claude tool-use chat engine, SSE streaming, chat history, multi-turn context. 36 tests. Merged to main.

### Deliverables
- chat_engine.py rewritten: keyword routing → Claude function calling
- SSE streaming endpoint /v1/chat/stream
- agent.chat_history table for multi-turn
- Security: off-by-one fix, error leakage prevention
