Update AI instructions with layout patterns and testing guidance

[wibjorn]

• css: Add CSS custom properties customization pattern and layout modifier class guidance
• integration: Add CSS component testing patterns and viewport-based test skipping docs
• site: Add scaffold templates and scripts section
• copilot-instructions: Mention HTML templates in scaffold description

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[Explain the context of this pull request here]

Testing

1. Step one
2. Step two
   Expected result:

Additional information

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Contributor checklist

• Did you update the description of this pull request with a review link and test steps?
• Did you submit a changeset? Changesets are required for all code changes.
• Does your pull request implement complex UI logic (js/ts)? Consider adding an integration test to test your user flow.
• Does your pull request add a new page to the documentation site? Add your new page for automated accessibility testing in /integration/tests/accessibility.
• Does your pull request change any transforms? Did you test they work on right to left?