Q170: agent model suggestions and choices follow the picked provider

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shared/agent-model.ts

@@ -9,6 +9,11 @@ export function agentModel(agentModelId: string | null, providerModel: string |
   return { model: null, source: "none" };
 }
 
+/** A model names one of its provider's models, so it means nothing without that provider. */
+export function assertModelHasProvider(modelProviderId: string | null, modelId: string | null): void {
+  if (modelId?.trim() && !modelProviderId) throw new Error("Pick the provider before its model.");
+}
+
 /** Agents a change to this provider's default model reaches, and those that keep their own model. */
 export function defaultModelReach(agents: Array<{ modelProviderId: string | null; modelId: string | null }>, providerId: string): { following: number; ownModel: number } {
   const bound = agents.filter((agent) => agent.modelProviderId === providerId);