fix: clean up styles & colors and define in styles.md

[ae-openai]

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.)