Show persistent timestamps for operation and teacher messages

shared/coding-conversation-contracts.ts

@@ -142,6 +142,8 @@ export type ConversationContentBlock =
 export interface ConversationMessageNode {
   kind: 'message';
   id: string;
+  /** Epoch ms: user send time / assistant completion time. Absent for undated history. */
+  timestamp?: number;
   sourceEntryId?: string;
   clientRequestId?: string;
   role: 'user' | 'assistant';