[WIP] Set up Copilot instructions for repository

[Copilot]

• Research and understand best practices for Copilot coding agent instructions
• Review repository structure, coding standards, and development practices
• Create .github/copilot-instructions.md file with comprehensive instructions
• Verify the instructions file is properly formatted and complete
• Test that the file is accessible and properly placed

Summary

Created .github/copilot-instructions.md with comprehensive guidance for GitHub Copilot including:

• Project overview and architecture
• Language/framework specifications (R/Shiny)
• Code style standards (tidyverse style guide, 120 char line length)
• Development workflow (R CMD check, roxygen2, linting)
• Testing patterns (testthat, shinytest2)
• Common architectural patterns (teal modules, data handling, filtering, reporting)
• File organization conventions
• Security and contribution guidelines
• Useful resources and links

The instructions align with the repository's existing practices documented in CONTRIBUTING.md and reflect the actual code structure and dependencies.

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

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

Fixes #1621

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.