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/onboarding/mod.rs

@@ -1,3 +1,5 @@
 mod auth;
+mod continue_to_chat;
+mod git_warning;
 pub mod onboarding_screen;
 mod welcome;