Elaborate on KTLO mode warning in README

[lesliecdubs]

What are you trying to accomplish?

Update warning message to clarify preferred usage of primer/css and utility classes

Can these changes ship as is?

• Yes, this PR does not depend on additional changes. 🚢

[changeset-bot]

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Latest commit: 65bc27f

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[Copilot]

Pull Request Overview

This PR updates the KTLO (Keep The Lights On) mode warning in the README to provide clearer guidance on when and how to use primer/css utility classes versus the preferred component libraries.

Key Changes

• Clarifies that existing utility classes from primer/css can still be used where needed
• Better explains the preference hierarchy: primer/react first, then primer/view_components if necessary

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[lukasoppermann]

👍