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/app-server-protocol/src/protocol/thread_history.rs

@@ -2002,6 +2002,8 @@ mod tests {
                     .expect("valid sender thread id"),
                 receiver_thread_id: ThreadId::try_from("00000000-0000-0000-0000-000000000002")
                     .expect("valid receiver thread id"),
+                receiver_agent_nickname: None,
+                receiver_agent_role: None,
                 status: AgentStatus::Completed(None),
             }),
         ];