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/markdown.rs

@@ -2,7 +2,6 @@ use crate::citation_regex::CITATION_REGEX;
 use codex_core::config::Config;
 use codex_core::config_types::UriBasedFileOpener;
 use ratatui::text::Line;
-use ratatui::text::Span;
 use std::borrow::Cow;
 use std::path::Path;
 
@@ -44,7 +43,7 @@ fn append_markdown_with_opener_and_cwd(
                     } else {
                         line
                     };
-                    let owned_line: Line<'static> = Line::from(Span::raw(line.to_string()));
+                    let owned_line: Line<'static> = line.to_string().into();
                     lines.push(owned_line);
                 }
             }