Connecting to the bigger picture

This executor wraps Chapter 6's registry and Chapter 7's gated_call, so parallel calls are still validated and approved. The items/turns/threads vocabulary from the App Server is the same structure Chapter 3 used for history and Chapter 11 will use for subagent communication. And the stateless-plus-caching tradeoff ties directly back to Chapter 5: re-sending everything is only viable because the prefix is cached. In the capstone, the loop uses run_tools whenever the model requests more than one call at once.