feat(coding): keep one ongoing conversation per delivered agent

electron/coding-teacher/store.ts

@@ -101,7 +101,7 @@ export class TeacherTopicStore {
     }
     return await pending;
   }
-  async save(topic: TeacherTopic) {
+  async save(topic: TeacherTopic, _requestId?: string) {
     await mkdir(this.directory, { recursive: true });
     const saved = { ...topic };
     delete saved.unsaved;