[config] Onboarding flow with persistence (#1929)

## Summary
In collaboration with @gpeal: upgrade the onboarding flow, and persist
user settings.

---------

Co-authored-by: Gabriel Peal <gabriel@openai.com>

codex-rs/exec/src/lib.rs

@@ -181,7 +181,7 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
     event_processor.print_config_summary(&config, &prompt);
 
     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.");
+        eprintln!("Not inside a trusted directory and --skip-git-repo-check was not specified.");
         std::process::exit(1);
     }