Rename agent/route to agent/model and make its result a complete
ModelSelection, so each step fixes its provider, model, and reasoning
effort before prompt preparation, agent/pre-step, tool conditions, and
every request attempt. agent/request may change only sampling controls;
returning another provider, model, or reasoning effort fails the step.
installModelSelection installs one agent/model listener and
ModelSelectionRef drops assembled. The pre-step payload, AssembleContext,
and ToolActivationContext carry modelSelection. Webhook initial
selection and the CLI mock effort switch move to agent/model.