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';

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[];

shared/message-timestamp.ts

@@ -0,0 +1,7 @@
+/** Read only real, valid epoch milliseconds / ISO dates; never invent a history time. */
+export function messageTimestamp(value: unknown): number | undefined {
+  const timestamp = typeof value === 'number' ? value
+    : typeof value === 'string' && /^\d{4}-\d{2}-\d{2}T/.test(value) ? Date.parse(value) : NaN;
+  return Number.isFinite(timestamp) && timestamp >= 0 && !Number.isNaN(new Date(timestamp).getTime())
+    ? timestamp : undefined;
+}