feat(agent): choose provider, model, and effort through agent/model

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.

scripts/fixtures/python-sdk-dynamic-tools.mjs

@@ -14,7 +14,7 @@ export function apply(ctx) {
       render(_args, value) { return [{ type: 'text', text: value }] },
     },
     async execute() { return 'pong' },
-  }, { when: async ({ route }) => active && route.provider === 'deepseek-official' })
+  }, { when: async ({ modelSelection }) => active && modelSelection.provider === 'deepseek-official' })
   ctx.on('tools/post-execute', async (exec, result, next) => {
     const downstream = await next()
     if (result.isError) return downstream