Add Copilot code review instructions

[Copilot]

Adds .github/copilot-instructions.md to enable automatic Copilot code review with repository-specific guidance.

Review Guidelines

The instructions file covers:

• R code standards — roxygen2 docs, NAMESPACE exports, parameter typing
• htmlwidgets/JS integration — D3.js practices, XSS prevention, JSON serialization
• Shiny integration — reactive efficiency, input validation
• Testing/docs — coverage expectations, NEWS.md updates
• Compatibility — backward compat, R version, htmlwidgets compat

Reference: GitHub blog on Copilot Code Review instructions

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE] add copilot review to be run automatically</issue_title>
<issue_description>Some context:

https://github.blog/ai-and-ml/unlocking-the-full-power-of-copilot-code-review-master-your-instructions-files/</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [FEATURE] add copilot review to be run automatically #93

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