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/core/src/agent/control/residency_tests.rs

@@ -159,6 +159,7 @@ async fn mark_thread_completed(thread: &CodexThread) {
             turn.as_ref(),
             EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: turn.sub_id.clone(),
+                started_at: None,
                 last_agent_message: Some("done".to_string()),
                 completed_at: None,
                 duration_ms: None,
@@ -178,6 +179,7 @@ async fn mark_thread_interrupted(thread: &CodexThread) {
             turn.as_ref(),
             EventMsg::TurnAborted(TurnAbortedEvent {
                 turn_id: Some(turn.sub_id.clone()),
+                started_at: None,
                 reason: TurnAbortReason::Interrupted,
                 completed_at: None,
                 duration_ms: None,