- Add buildDeepSeekProvider() with deepseek-chat and deepseek-reasoner models
- Register in resolveImplicitProviders() for auto-discovery via DEEPSEEK_API_KEY
- Add DEEPSEEK_API_KEY to env var mapping in model-auth.ts
- Add unit tests for provider builder and implicit resolution
- Add documentation at docs/providers/deepseek.md
Closes#3393
