Add Copilot instructions targeting .NET 9 instead of .NET 8

[Copilot]

This PR adds comprehensive Copilot instructions to guide development with .NET 9 as the primary target framework instead of .NET 8.

What's Added

A new .copilot-instructions.md file that provides:

• Framework targeting guidelines specifying .NET 9 as the primary target (net9.0, net9.0-windows, etc.)

• Platform-specific guidance for all UI frameworks used in the repository:

  • WinUI projects: net9.0-windows10.0.19041.0
  • WPF projects: net9.0-windows
  • Avalonia projects: net9.0 for cross-platform
  • MAUI projects: net9.0 with platform-specific TFMs
  • Uno Platform projects: .NET 9 compatibility guidelines

• Modern development patterns leveraging .NET 9 features:

  • Primary constructors for cleaner code
  • Collection expressions for better performance
  • File-scoped namespaces for organization
  • Record types for data containers

• Codebreaker-specific examples showing .NET 9 patterns for dependency injection and ViewModels

Alignment with Repository Guidelines

The instructions extend the existing guidelines.md while maintaining consistency with the repository's focus on:

• Modern technologies (.NET 9 as the latest stable release)
• Productivity through built-in .NET features
• Using released versions for developer accessibility

Migration Guidance

Includes step-by-step guidance for updating existing projects from .NET 8 to .NET 9, covering project files, package references, and testing considerations.

Fixes #200.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@christiannagel 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.