✨ Set up comprehensive Copilot instructions with clarified .NET version requirements

[Copilot]

Overview

This PR completes the setup of Copilot instructions for the MaterialDesignInXamlToolkit repository as requested in the issue, following GitHub's best practices for Copilot coding agents (https://gh.io/copilot-coding-agent-tips).

What's Included

The .github/copilot-instructions.md file provides comprehensive guidance covering:

• Repository Overview: Clear explanation that this is a theme library (not a control library) for WPF applications implementing Material Design
• Architecture & Structure: Documentation of all core projects including MaterialDesignThemes.Wpf, MaterialDesignColors.Wpf, and demo applications
• Development Environment: Complete setup requirements and tooling information
• Build & Test Instructions: Working commands for restore, build, test, and package creation
• Code Style & Conventions: C# and XAML style guidelines including dependency property patterns and resource naming
• Domain Knowledge: WPF theming best practices, Material Design implementation guidelines, and accessibility requirements
• Common Patterns: Real code examples for theme modifications, style additions, and control development
• Build Pipeline: Documentation of GitHub Actions workflows and PowerShell automation scripts

Key Improvements Made

Enhanced the accuracy of version requirements to prevent confusion:

1. Clarified .NET versions: Distinguished between the .NET 9 SDK requirement (for building, as specified in global.json) and the actual target frameworks (.NET 8 and .NET Framework 4.7.2) that the library supports

2. Updated Visual Studio reference: Changed from generic "Visual Studio" to specific "Visual Studio 2022" to match the README and actual requirements

The distinction is important because developers need .NET 9 SDK installed to build the project, but the library itself targets .NET 8 and .NET Framework 4.7.2 for broader compatibility.

Verification

All references in the instructions have been verified:

• ✅ PowerShell scripts exist (BuildNugets.ps1, ApplyXamlStyler.ps1, MigrateBrushes.ps1, UpdateNugets.ps1)
• ✅ GitHub Actions workflows exist (pr_verification.yml, build_artifacts.yml, release.yml)
• ✅ Project names and structure match the repository
• ✅ Version numbers match configuration files

This comprehensive instruction file will help both human developers and Copilot agents understand the repository's architecture, conventions, and best practices when contributing to or working with the MaterialDesignInXamlToolkit.

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 #3947

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