First pass at a TUI onboarding (#1876)

This sets up the scaffolding and basic flow for a TUI onboarding
experience. It covers sign in with ChatGPT, env auth, as well as some
safety guidance.

Next up:
1. Replace the git warning screen
2. Use this to configure default approval/sandbox modes


Note the shimmer flashes are from me slicing the video, not jank.

https://github.com/user-attachments/assets/0fbe3479-fdde-41f3-87fb-a7a83ab895b8

codex-rs/tui/src/main.rs

@@ -22,7 +22,9 @@ fn main() -> anyhow::Result<()> {
             .raw_overrides
             .splice(0..0, top_cli.config_overrides.raw_overrides);
         let usage = run_main(inner, codex_linux_sandbox_exe).await?;
-        println!("{}", codex_core::protocol::FinalOutput::from(usage));
+        if !usage.is_zero() {
+            println!("{}", codex_core::protocol::FinalOutput::from(usage));
+        }
         Ok(())
     })
 }