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/onboarding/welcome.rs

@@ -0,0 +1,23 @@
+use ratatui::buffer::Buffer;
+use ratatui::layout::Rect;
+use ratatui::prelude::Widget;
+use ratatui::style::Modifier;
+use ratatui::style::Style;
+use ratatui::text::Line;
+use ratatui::text::Span;
+use ratatui::widgets::WidgetRef;
+
+pub(crate) struct WelcomeWidget {}
+
+impl WidgetRef for &WelcomeWidget {
+    fn render_ref(&self, area: Rect, buf: &mut Buffer) {
+        let line = Line::from(vec![
+            Span::raw("> "),
+            Span::styled(
+                "Welcome to Codex, OpenAI's coding agent that runs in your terminal",
+                Style::default().add_modifier(Modifier::BOLD),
+            ),
+        ]);
+        line.render(area, buf);
+    }
+}