Fall back to configured instruction files if AGENTS.md isn't available (#4544)

Allow users to configure an agents.md alternative to consume, but warn
the user it may degrade model performance.

Fixes #4376

codex-rs/tui/src/chatwidget/tests.rs

@@ -2,6 +2,7 @@ use super::*;
 use crate::app_event::AppEvent;
 use crate::app_event_sender::AppEventSender;
 use crate::test_backend::VT100Backend;
+use crate::tui::FrameRequester;
 use codex_core::AuthManager;
 use codex_core::CodexAuth;
 use codex_core::config::Config;