Add a distinct simple_model field alongside ai_model so operators can route cheap, simple tasks (e.g. email-label translation) to a low-cost model while keeping the strong model for parent conversations. The two models can be from different providers (e.g. Gemini + Haiku). If SIMPLE_MODEL is unset, falls back to AI_MODEL with a warning. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>