introduce separate SIMPLE_MODEL config for lightweight tasks

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>

main.py

@@ -61,6 +61,7 @@ def build_components(config: Config):
         indoor_email=config.admin_email_indoor,
         outdoor_email=config.admin_email_outdoor,
         cc_emails=[e.strip() for e in config.admin_email_cc.split(",") if e.strip()],
+        model=config.simple_model,
     )
 
     agent = EmailAgent(