feat(session): add per-message created_at timestamps

- Persistence layer (jsonl.go addMsg/SetHistory) normalizes CreatedAt
  when missing so the invariant is guaranteed at the storage boundary
- API layer (session.go) exposes created_at on all transcript message
  types with session.updated fallback for legacy messages
- Frontend uses per-message timestamps when available
- messagesContentEqual ignores CreatedAt for tail-matching after
  JSONL roundtrip

Fixes #2787

pkg/agent/turn_state.go

@@ -686,10 +686,25 @@ func (ts *turnState) restoreSession(agent *AgentInstance) error {
 	return agent.Sessions.Save(ts.sessionKey)
 }
 
+// messagesContentEqual compares two message slices by content only, ignoring CreatedAt.
+// JSON roundtrip loses the monotonic clock portion of time.Time, so direct
+// reflect.DeepEqual would always differ on messages that roundtripped through
+// the JSONL store.
+func messagesContentEqual(a, b []providers.Message) bool {
+	for i := range a {
+		aCopy, bCopy := a[i], b[i]
+		aCopy.CreatedAt, bCopy.CreatedAt = nil, nil
+		if !reflect.DeepEqual(aCopy, bCopy) {
+			return false
+		}
+	}
+	return true
+}
+
 func matchingTurnMessageTail(history, persisted []providers.Message) int {
 	maxMatch := min(len(history), len(persisted))
 	for size := maxMatch; size > 0; size-- {
-		if reflect.DeepEqual(history[len(history)-size:], persisted[len(persisted)-size:]) {
+		if messagesContentEqual(history[len(history)-size:], persisted[len(persisted)-size:]) {
 			return size
 		}
 	}