Added LLM routing module to cowork-service:
- lib/llm-router.ts — singleton LlmRouter with cloud + local Ollama support
- modules/llm/types.ts — Zod request schemas
- modules/llm/routes.ts — POST /api/llm/chat, GET /api/llm/providers, GET /api/llm/health
- All endpoints gated by llm_multi_model_enabled feature flag
- Best-effort init: service works without API keys (router stays uninitialized)
- 8 new tests (routes), server test updated for 3 route modules
- 57 total tests passing, typecheck clean