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/core/src/protocol.rs

@@ -429,6 +429,12 @@ pub struct TokenUsage {
     pub total_tokens: u64,
 }
 
+impl TokenUsage {
+    pub fn is_zero(&self) -> bool {
+        self.total_tokens == 0
+    }
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize)]
 pub struct FinalOutput {
     pub token_usage: TokenUsage,