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.

AGENTS.md

@@ -12,6 +12,10 @@ Before finalizing a change to `codex-rs`, run `just fmt` (in `codex-rs` director
 1. Run the test for the specific project that was changed. For example, if changes were made in `codex-rs/tui`, run `cargo test -p codex-tui`.
 2. Once those pass, if any changes were made in common, core, or protocol, run the complete test suite with `cargo test --all-features`.
 
+## TUI style conventions
+
+See `codex-rs/tui/styles.md`.
+
 ## TUI code conventions
 
 - Use concise styling helpers from ratatui’s Stylize trait.