Pass initial history as an optional to codex delegate (#5950)

This will give us more freedom on controlling the delegation. i.e we can
fork our history and run `compact`.

codex-rs/core/src/tasks/review.rs

@@ -90,6 +90,7 @@ async fn start_review_conversation(
         session.clone_session(),
         ctx.clone(),
         cancellation_token,
+        None,
     )
     .await)
         .ok()