Add GitHub Copilot instructions file for automated code review

[Copilot]

Adds .github/copilot-instructions.md to enable context-aware Copilot code reviews as requested in the linked GitHub blog post.

Changes

• Created .github/copilot-instructions.md with R package-specific review guidelines covering:
  • roxygen2 documentation standards (@param, @return, @examples)
  • Backward compatibility requirements for visualization functions
  • testthat testing expectations
  • Key files to focus on (R/heatmaply.R, R/heatmapr.R, R/ggheatmap.R)

To enable

Repository owner must enable "Copilot code review" in Settings → Code security and analysis. The instructions file will then be automatically consumed during PR reviews.

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 #313

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.