How do I structure my first Catalyst durable agent project so that the business logic remains testable independently of the durable execution runtime?

Separate agent logic into plain functions and orchestrate them with Catalyst workflows to keep unit testing lightweight. Encapsulate business decisions in pure handlers that receive state, and use the workflow definition only for ordering steps and error recovery; test handlers with mocks. This separation allows fast local iteration without the execution engine. Be aware that subtle coupling can arise if workflow context leaks into handlers, so review shared objects carefully.