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/exec/src/lib.rs

@@ -180,7 +180,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     // is using.
     event_processor.print_config_summary(&config, &prompt);
 
-    if !skip_git_repo_check && !is_inside_git_repo(&config) {
+    if !skip_git_repo_check && !is_inside_git_repo(&config.cwd.to_path_buf()) {
         eprintln!("Not inside a Git repo and --skip-git-repo-check was not specified.");
         std::process::exit(1);
     }