Add Copilot code review instructions

.github/copilot-instructions.md

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+# Copilot Code Review Instructions
+
+## Repository Overview
+
+The `installr` package is an R package designed for Windows that automates the installation and updating of software, with a special focus on R itself. The package provides:
+
+1. Easy R updates on Windows via the `updateR()` function
+2. Installation functions for R development software (git, Rtools, etc.)
+3. Installation functions for reproducible research tools (MikTeX, pandoc, etc.)
+
+## Code Style Guidelines
+
+- Follow R best practices and conventions
+- Use roxygen2 documentation for all exported functions
+- Line length should not exceed 100 characters (see `.lintr` configuration)
+- Functions should handle Windows-specific paths and URLs appropriately
+
+## Testing Guidelines
+
+- Tests are located in the `tests/` directory and use the `testthat` framework
+- Many functions are Windows-only, so tests should be skipped on non-Windows platforms when appropriate
+- Mock external downloads and web requests in tests
+
+## Review Focus Areas
+
+When reviewing code changes, please pay special attention to:
+
+1. **URL handling**: Ensure download URLs are valid and use HTTPS where possible
+2. **Windows compatibility**: Functions should work correctly on Windows operating systems
+3. **Error handling**: Proper error messages and graceful failure handling
+4. **Documentation**: All exported functions should have complete roxygen2 documentation
+5. **Backwards compatibility**: Changes should not break existing functionality
+
+## Dependencies
+
+- Core dependencies: `stringr`, `utils`
+- Suggested packages are used for optional functionality (see DESCRIPTION file)
+- Minimize adding new dependencies when possible