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/tests/suite/vt100_streaming_no_dup.rs

@@ -2,7 +2,6 @@
 
 use ratatui::backend::TestBackend;
 use ratatui::layout::Rect;
-use ratatui::text::Line;
 
 fn term(viewport: Rect) -> codex_tui::custom_terminal::Terminal<TestBackend> {
     let backend = TestBackend::new(20, 6);
@@ -23,7 +22,7 @@ fn stream_commit_trickle_no_duplication() {
     codex_tui::insert_history::insert_history_lines_to_writer(
         &mut t,
         &mut out1,
-        vec![Line::from("one")],
+        vec!["one".into()],
     );
 
     // Step 2: later commit next row
@@ -31,7 +30,7 @@ fn stream_commit_trickle_no_duplication() {
     codex_tui::insert_history::insert_history_lines_to_writer(
         &mut t,
         &mut out2,
-        vec![Line::from("two")],
+        vec!["two".into()],
     );
 
     let combined = [out1, out2].concat();
@@ -62,7 +61,7 @@ fn live_ring_rows_not_inserted_into_history() {
     codex_tui::insert_history::insert_history_lines_to_writer(
         &mut t,
         &mut buf,
-        vec![Line::from("one"), Line::from("two")],
+        vec!["one".into(), "two".into()],
     );
 
     // The live ring might display tail+head rows like ["two", "three"],