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.rs

@@ -3,6 +3,7 @@ use codex_file_search::FileMatch;
 use crossterm::event::KeyEvent;
 use ratatui::text::Line;
 
+use crate::app::ChatWidgetArgs;
 use crate::slash_command::SlashCommand;
 
 #[allow(clippy::large_enum_variant)]
@@ -51,4 +52,5 @@ pub(crate) enum AppEvent {
 
     /// Onboarding: result of login_with_chatgpt.
     OnboardingAuthComplete(Result<(), String>),
+    OnboardingComplete(ChatWidgetArgs),
 }