Show persistent timestamps for operation and teacher messages

shared/coding-teacher.ts

@@ -96,6 +96,8 @@ export interface TeacherRequest {
   text: string;
   references: TeacherReference[];
   createdAt: string;
+  /** Actual terminal time, absent in older history; never inferred from topic.updatedAt. */
+  finishedAt?: string;
   sourceCursor: TeacherSourceContext['cursor'];
   sourceCapturedAt: string;
   includedSourceMessageIds: string[];