prefer ratatui Stylized for constructing lines/spans (#3068)

no functional change, just simplifying ratatui styling and adding
guidance in AGENTS.md for future.

codex-rs/tui/src/onboarding/welcome.rs

@@ -1,10 +1,8 @@
 use ratatui::buffer::Buffer;
 use ratatui::layout::Rect;
 use ratatui::prelude::Widget;
-use ratatui::style::Modifier;
-use ratatui::style::Style;
+use ratatui::style::Stylize;
 use ratatui::text::Line;
-use ratatui::text::Span;
 use ratatui::widgets::WidgetRef;
 
 use crate::onboarding::onboarding_screen::StepStateProvider;
@@ -18,11 +16,8 @@ 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 command-line coding agent",
-                Style::default().add_modifier(Modifier::BOLD),
-            ),
+            ">_ ".into(),
+            "Welcome to Codex, OpenAI's command-line coding agent".bold(),
         ]);
         line.render(area, buf);
     }