fix: clean up styles & colors and define in styles.md (#2401)

New style guide:

  # Headers, primary, and secondary text
  
- **Headers:** Use `bold`. For markdown with various header levels,
leave in the `#` signs.
  - **Primary text:** Default.
  - **Secondary text:** Use `dim`.
  
  # Foreground colors
  
- **Default:** Most of the time, just use the default foreground color.
`reset` can help get it back.
- **Selection:** Use ANSI `blue`. (Ed & AE want to make this cyan too,
but we'll do that in a followup since it's riskier in different themes.)
  - **User input tips and status indicators:** Use ANSI `cyan`.
  - **Success and additions:** Use ANSI `green`.
  - **Errors, failures and deletions:** Use ANSI `red`.
  - **Codex:** Use ANSI `magenta`.
  
  # Avoid
  
- Avoid custom colors because there's no guarantee that they'll contrast
well or look good on various terminal color themes.
- Avoid ANSI `black`, `white`, `yellow` as foreground colors because the
terminal theme will do a better job. (Use `reset` if you need to in
order to get those.) The exception is if you need contrast rendering
over a manually colored background.
  
  (There are some rules to try to catch this in `clippy.toml`.)

# Testing

Tested in a variety of light and dark color themes in Terminal, iTerm2, and Ghostty.

codex-rs/tui/src/lib.rs

@@ -2,6 +2,7 @@
 // The standalone `codex-tui` binary prints a short help message before the
 // alternate‑screen mode starts; that file opts‑out locally via `allow`.
 #![deny(clippy::print_stdout, clippy::print_stderr)]
+#![deny(clippy::disallowed_methods)]
 use app::App;
 use codex_core::BUILT_IN_OSS_MODEL_PROVIDER_ID;
 use codex_core::config::Config;
@@ -28,7 +29,6 @@ mod bottom_pane;
 mod chatwidget;
 mod citation_regex;
 mod cli;
-mod colors;
 mod common;
 pub mod custom_terminal;
 mod diff_render;