Update a11y instructions with latest optimizations

[mfairchild365]

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 plugin.
• New skill file.
• Update to existing instruction, prompt, agent, plugin, 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 repository’s accessibility instruction set (instructions/a11y.instructions.md) to reflect newer guidance around component-library usage, keyboard/focus behavior, forced-colors support, and reflow expectations.

Changes:

• Refines “non‑negotiables” guidance for UI component libraries and contrast expectations.
• Adds/adjusts keyboard/focus and aria-activedescendant guidance, plus a forced-colors/focus-style CSS example.
• Rewords WCAG reflow guidance (320px) and expands navigation/sub-navigation guidance.

[aaronpowell]

Can you repoint this branch to be staged as the contributing docs. Here's how to do that with the GitHub CLI:

gh pr edit 767 --base staged