Include start times in terminal turn events (#32263)

## What changed

- Add an optional `started_at` Unix timestamp to `TurnCompleteEvent` and
  `TurnAbortedEvent`.
- Populate the timestamp from turn timing state when a turn completes or is
  aborted.
- Preserve the timestamp when synthesizing interrupted fork history and when
  importing external sessions.

## Testing

- Verify that interrupting a turn emits a `TurnAbortedEvent` with a start
  timestamp.

GitOrigin-RevId: 75b3911b95c060e5fc1843a5e293d8fe47377186

codex-rs/thread-store/src/local/mod.rs

@@ -538,6 +538,7 @@ mod tests {
                 )),
                 RolloutItem::EventMsg(EventMsg::TurnComplete(TurnCompleteEvent {
                     turn_id: "turn-1".to_string(),
+                    started_at: None,
                     last_agent_message: None,
                     completed_at: None,
                     duration_ms: None,