✨ Add Copilot instructions for React Native Windows samples repository

[Copilot]

This PR implements comprehensive Copilot instructions for the react-native-windows-samples repository to help GitHub Copilot better understand the codebase structure, development patterns, and best practices.

What's Added

Created .github/copilot-instructions.md with detailed guidance covering:

• Repository Overview: Purpose and relationship to main React Native Windows/macOS repos
• Technology Stack: React Native Windows 0.79+, C++/WinRT, C#, TypeScript/JavaScript
• Repository Structure: Samples organization, legacy code, documentation, and CI/CD
• Development Environment: Prerequisites, setup instructions, and common commands
• Sample Projects: Detailed information about Calculator, NativeModuleSample, and ContinuousIntegration samples
• Build System: GitHub Actions workflows, build matrix (Debug/Release, x86/x64/ARM64)
• Code Patterns: File structure conventions, naming patterns, and coding standards
• Testing Approach: Jest configuration, native code testing, and integration testing
• Common Tasks: Adding samples, upgrading versions, working with native code
• Troubleshooting: Solutions for build failures, Metro issues, and native module problems
• Best Practices: Guidelines for contributors and cross-platform development

Benefits

These instructions will help Copilot:

• Generate more accurate code suggestions for React Native Windows development
• Understand the multi-language nature of the repository (JS/TS, C++, C#)
• Provide better guidance for native module development
• Suggest appropriate build commands and project structure
• Offer relevant troubleshooting advice

The instructions follow GitHub's best practices for Copilot coding agent configuration and are tailored specifically to the unique aspects of React Native Windows development.

Fixes #1074.

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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.

Microsoft Reviewers: Open in CodeFlow