feat: cleaner TUI for sub-agents (#12327)

<img width="760" height="496" alt="Screenshot 2026-02-20 at 14 31 25"
src="https://github.com/user-attachments/assets/1983b825-bb47-417e-9925-6f727af56765"
/>

codex-rs/exec/tests/event_processor_with_json_output.rs

@@ -572,6 +572,8 @@ fn collab_spawn_begin_and_end_emit_item_events() {
             call_id: "call-10".to_string(),
             sender_thread_id,
             new_thread_id: Some(new_thread_id),
+            new_agent_nickname: None,
+            new_agent_role: None,
             prompt: prompt.clone(),
             status: AgentStatus::Running,
         }),
@@ -623,6 +625,7 @@ fn collab_wait_end_without_begin_synthesizes_failed_item() {
         EventMsg::CollabWaitingEnd(CollabWaitingEndEvent {
             sender_thread_id,
             call_id: "call-11".to_string(),
+            agent_statuses: Vec::new(),
             statuses: statuses.clone(),
         }),
     );