syntax-highlight bash lines (#3142)

i'm not yet convinced i have the best heuristics for what to highlight,
but this feels like a useful step towards something a bit easier to
read, esp. when the model is producing large commands.

<img width="669" height="589" alt="Screenshot 2025-09-03 at 8 21 56 PM"
src="https://github.com/user-attachments/assets/b9cbcc43-80e8-4d41-93c8-daa74b84b331"
/>

also a fairly significant refactor of our line wrapping logic.

AGENTS.md

@@ -37,7 +37,15 @@ See `codex-rs/tui/styles.md`.
 - Avoid churn: don’t refactor between equivalent forms (Span::styled ↔ set_style, Line::from ↔ .into()) without a clear readability or functional gain; follow file‑local conventions and do not introduce type annotations solely to satisfy .into().
 - Compactness: prefer the form that stays on one line after rustfmt; if only one of Line::from(vec![…]) or vec![…].into() avoids wrapping, choose that. If both wrap, pick the one with fewer wrapped lines.
 
-## Snapshot tests
+### Text wrapping
+- Always use textwrap::wrap to wrap plain strings.
+- If you have a ratatui Line and you want to wrap it, use the helpers in tui/src/wrapping.rs, e.g. word_wrap_lines / word_wrap_line.
+- If you need to indent wrapped lines, use the initial_indent / subsequent_indent options from RtOptions if you can, rather than writing custom logic.
+- If you have a list of lines and you need to prefix them all with some prefix (optionally different on the first vs subsequent lines), use the `prefix_lines` helper from line_utils.
+
+## Tests
+
+### Snapshot tests
 
 This repo uses snapshot tests (via `insta`), especially in `codex-rs/tui`, to validate rendered output. When UI or text output changes intentionally, update the snapshots as follows:
 
@@ -52,3 +60,7 @@ This repo uses snapshot tests (via `insta`), especially in `codex-rs/tui`, to va
 
 If you don’t have the tool:
 - `cargo install cargo-insta`
+
+### Test assertions
+
+- Tests should use pretty_assertions::assert_eq for clearer diffs. Import this at the top of the test module if it isn't already.