Add comprehensive copilot-instructions.md for enhanced GitHub Copilot assistance

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to improve GitHub Copilot's understanding of the DeviceRunners repository structure, conventions, and development patterns.

What's Added

The copilot-instructions.md file provides detailed guidance on:

Project Architecture

• Multi-platform device testing framework supporting Android, iOS, macOS (Catalyst), and Windows
• Core components overview: DeviceRunners.Core, VisualRunners, XHarness, UITesting, and CLI tools
• Testing framework integration for Xunit and NUnit with both visual and CLI runners

Development Conventions

• Platform-specific code organization using Platforms/ folder structure with conditional compilation
• Service-oriented architecture patterns demonstrated in the CLI tool
• MVVM patterns for visual runners with ObservableCollection and ICommand usage
• Command patterns for CLI tools using Spectre.Console

Technical Guidelines

• .NET 9.0 multi-targeting configuration and MSBuild conventions
• Coding standards aligned with .editorconfig (tabs for C#, spaces for XML)
• Testing patterns for device tests, CLI tools, and visual components
• Build system conventions using Central Package Management and artifacts output

Common Development Scenarios

• Adding new testing framework support
• Extending platform capabilities
• Creating CLI tool commands
• Implementing MAUI integrations

Example Usage Context

The instructions help Copilot understand patterns like:

// Platform-specific implementations
#if ANDROID
    return Android.App.Application.Context.CacheDir.AbsolutePath;
#elif WINDOWS
    if (IsPackagedApp.Value)
        return Windows.ApplicationModel.Package.Current.InstalledLocation.Path;
#endif

// Service injection patterns
public class PackageService
{
    public Task<bool> IsPackageInstalled(string identity) { }
}

// MVVM patterns for visual runners
public class HomeViewModel : AbstractBaseViewModel
{
    public ObservableCollection<TestAssemblyViewModel> TestAssemblies { get; }
    public ICommand RunEverythingCommand { get; }
}

This will enable GitHub Copilot to provide more accurate suggestions when working with this complex multi-platform testing framework, understanding the project's unique architecture and development patterns.

Fixes #78.

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