✨ Set up Copilot instructions for MaterialDesignInXamlToolkit

[Copilot]

This PR adds comprehensive Copilot instructions for the MaterialDesignInXamlToolkit repository following GitHub's best practices for Copilot coding agent.

The new .github/copilot-instructions.md file provides essential guidance for AI-assisted development, covering:

Key Areas Covered

Repository Context

• Clear identification as a WPF theming library (not a control library)
• Core principles emphasizing styling existing WPF controls over creating new ones
• Architecture overview of main projects and their purposes

Development Environment

• Windows compilation requirements for WPF development
• .NET 9 SDK dependency and build commands
• PowerShell automation scripts usage

Code Style & Conventions

• C# and XAML formatting rules from .editorconfig
• Dependency property patterns and resource naming conventions
• XamlStyler configuration integration

Domain-Specific Knowledge

• Material Design implementation guidelines
• WPF theming best practices and patterns
• Theme modification using PaletteHelper and color adjustments
• Resource dictionary organization and naming

Build & Test Pipeline

• GitHub Actions workflow integration
• Testing approach with unit tests and UI tests
• Build automation with PowerShell scripts

API Design Guidelines

• Backward compatibility requirements
• Minimal public API surface principles
• Designer support considerations

This comprehensive guide will help Copilot understand the repository's specific context, development patterns, and WPF/Material Design domain knowledge to provide more accurate and helpful code suggestions.

Fixes #3909.

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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.