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/core/src/util.rs

@@ -1,3 +1,4 @@
+use std::path::Path;
 use std::sync::Arc;
 use std::time::Duration;
 
@@ -5,8 +6,6 @@ use rand::Rng;
 use tokio::sync::Notify;
 use tracing::debug;
 
-use crate::config::Config;
-
 const INITIAL_DELAY_MS: u64 = 200;
 const BACKOFF_FACTOR: f64 = 1.3;
 
@@ -47,8 +46,8 @@ pub(crate) fn backoff(attempt: u64) -> Duration {
 /// `git worktree add` where the checkout lives outside the main repository
 /// directory. If you need Codex to work from such a checkout simply pass the
 /// `--allow-no-git-exec` CLI flag that disables the repo requirement.
-pub fn is_inside_git_repo(config: &Config) -> bool {
-    let mut dir = config.cwd.to_path_buf();
+pub fn is_inside_git_repo(base_dir: &Path) -> bool {
+    let mut dir = base_dir.to_path_buf();
 
     loop {
         if dir.join(".git").exists() {