Decides whether a turn is routine enough for the local model or needs the cloud.

The decision is a pure function of the input and the history, so it is deterministic and testable without a network. Local is chosen only when the input positively looks routine; any doubt goes to the cloud, where the quality is. The rules are applied in this order:

1. an input that is not a string, or not valid UTF-8, is :cloud, because it cannot be judged;
2. an input longer than 280 characters is :cloud;
3. a history of more than 20 turns is :cloud, because the context is heavy;
4. an input that asks for reasoning or writing is :cloud;
5. an input that asks for triage or formatting is :local;
6. anything else is :cloud.