Revise accessibility instructions for clarity and detail

[mfairchild365]

Updated accessibility instructions to improve clarity and specificity regarding coding practices for accessibility, including keyboard navigation and semantic structure.

I've benchmarked this new version against the old one across many LLMs and it is over 2x more effective.

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• My contribution adds a new instruction, prompt, agent, or skill file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, or skill with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New collection file.
• New skill file.
• Update to existing instruction, prompt, agent, collection or skill.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

Updates the existing accessibility instruction set (a11y.instructions.md) to be more structured and explicit about practical a11y implementation requirements (keyboard/focus, semantics, contrast, forced-colors, reflow), aiming to improve instruction clarity and effectiveness.

Changes:

• Reorganized the document into “MUST/SHOULD” sections with a final verification checklist.
• Added/expanded guidance for forced-colors/high-contrast mode, reflow at 320px, and token-based color usage.
• Simplified and consolidated prior detailed sections (keyboard, forms, tables/grids, graphics) into clearer rule sets.