Migrate GitWarning to OnboardingScreen (#1915)

This paves the way to do per-directory approval settings
(https://github.com/openai/codex/pull/1912).

This also lets us pass in a Config/ChatWidgetArgs into onboarding which
can then mutate it and emit the ChatWidgetArgs it wants at the end which
may be modified by the said approval settings.

<img width="1180" height="428" alt="CleanShot 2025-08-06 at 19 30 55"
src="https://github.com/user-attachments/assets/4dcfda42-0f5e-4b6d-a16d-2597109cc31c"
/>

codex-rs/tui/src/app_event_sender.rs

@@ -4,7 +4,7 @@ use crate::app_event::AppEvent;
 
 #[derive(Clone, Debug)]
 pub(crate) struct AppEventSender {
-    app_event_tx: Sender<AppEvent>,
+    pub app_event_tx: Sender<AppEvent>,
 }
 
 impl AppEventSender {