Add color differentiation for user prompts vs Claude responses

Problem

In the CLI conversation view, user prompts and Claude responses blend together visually. When scrolling back through a long session, there is no quick visual anchor for "here is where I asked something" — the questions get buried in the surrounding response text, so finding a specific prompt/answer pair means reading rather than scanning.

Suggestion

Render user prompts in a visually distinct color (and/or expose the prompt color as a configurable theme token), so they stand out from Claude's responses when scrolling through conversation history.

Options that would each solve it:

• A distinct default color for the user prompt line(s).
• A configurable theme token (e.g. userPrompt) so people can pick a color that works with their terminal theme.
• Respect NO_COLOR / provide a non-color fallback (e.g. a marker or rule) for users who disable colors.

Related

This is a follow-up to #4385, which requested the same thing and was closed/locked without the feature landing.