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/guardian/tests.rs

@@ -2229,6 +2229,7 @@ async fn guardian_reused_trunk_ignores_stale_prior_turn_completion() -> anyhow::
             id: "stale-turn".to_string(),
             msg: EventMsg::TurnComplete(TurnCompleteEvent {
                 turn_id: "stale-turn".to_string(),
+                started_at: None,
                 last_agent_message: Some(
                     "{\"risk_level\":\"high\",\"user_authorization\":\"low\",\"outcome\":\"deny\",\"rationale\":\"stale guardian rationale\"}"
                         .to_string(),