Add Copilot code review instructions

[Copilot]

Adds .github/copilot-instructions.md to enable context-aware GitHub Copilot code reviews on pull requests.

Changes

• Created .github/copilot-instructions.md with repository-specific context:
  • Package overview (R package for Windows software installation)
  • Code style guidelines (roxygen2, 100-char line limit per .lintr)
  • Testing guidelines (testthat, Windows-only considerations)
  • Review focus areas: URL handling, Windows compatibility, error handling, documentation, backwards compatibility
  • Dependency constraints

Copilot will automatically use these instructions when reviewing PRs, providing more relevant feedback aligned with this codebase's conventions.

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 #195

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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.