✨ Add comprehensive Copilot instructions

[Copilot]

Configures GitHub Copilot coding agent instructions per best practices to guide AI behavior for this R package.

Changes

Enhanced .github/copilot-instructions.md (124 lines) with:

• Coding conventions: tidyverse style, roxygen2 requirements, explicit namespacing, CRAN compliance patterns
• Testing standards: testthat 3rd edition, 80% coverage threshold, test file naming conventions
• Security guidelines: temp file cleanup, permission handling, suggested package checks, input validation
• Development workflow: restart R → document → test → check cycle with specific commands
• Documentation patterns: roxygen2 block structure with complete working examples
• Package architecture: logging system internals, path resolution order, RStudio addin patterns, versioning scheme
• Code examples: suggested package usage, temp file handling, documentation templates
• Operational guidance: preferred libraries from existing dependencies, commit message format

Maintains existing reference to AGENTS.md as authoritative source while providing quick reference optimized for Copilot consumption.

Original prompt

---

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 ✨ Set up Copilot instructions #11

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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.