feat: add post-compaction sub-agent infos (#12774)

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/session_prefix.rs

@@ -12,3 +12,10 @@ pub(crate) fn format_subagent_notification_message(agent_id: &str, status: &Agen
     .to_string();
     SUBAGENT_NOTIFICATION_FRAGMENT.wrap(payload_json)
 }
+
+pub(crate) fn format_subagent_context_line(agent_id: &str, agent_nickname: Option<&str>) -> String {
+    match agent_nickname.filter(|nickname| !nickname.is_empty()) {
+        Some(agent_nickname) => format!("- {agent_id}: {agent_nickname}"),
+        None => format!("- {agent_id}"),
+    }
+}