Add Copilot instructions for repository guidance

[Copilot]

Overview

This PR adds a comprehensive .github/copilot-instructions.md file to provide GitHub Copilot with context-aware guidance when working with the Gibbon Git Server repository.

What's New

The Copilot instructions file includes:

• Project Overview: Clear description of Gibbon Git Server as a self-hosted Git server built with ASP.NET Core 8.0, modernized from the legacy Bonobo Git Server
• Technology Stack: Complete listing of frameworks and libraries including ASP.NET Core 8, Entity Framework Core, LibGit2Sharp, MSTest, NSubstitute, and SCSS compilation
• Project Structure: Detailed folder hierarchy explaining the purpose of each major directory (Controllers, Views, Models, Data, Services, Repositories, etc.)
• Coding Conventions:
  • Reference to Microsoft C# Style Guide
  • Naming conventions (interfaces with I prefix, PascalCase for classes/methods, camelCase with underscore for private fields)
  • Code style rules from .editorconfig (4-space tabs, no trailing whitespaces, consistent formatting)
• Build and Test Commands: Ready-to-use commands for dotnet restore, dotnet build, dotnet test, dotnet publish, and EF migrations
• Architectural Patterns: MVC pattern, Repository pattern, Service layer, Dependency Injection, and Entity Framework code-first approach
• Testing Practices: MSTest framework usage, NSubstitute for mocking, test file naming conventions
• Authentication & Security: Cookie authentication for web, Basic authentication for Git operations, role-based authorization
• Configuration: Information about appsettings.json usage (not web.config)
• Important Notes: Cross-platform support, SQLite default database, active development status
• Links: References to contributing guidelines, documentation website, and GitHub repository

Benefits

With this file in place, GitHub Copilot will:

• Provide more accurate code suggestions that follow the project's conventions
• Understand the architectural patterns used in the codebase
• Generate code that adheres to the Microsoft C# Style Guide
• Suggest appropriate test patterns using MSTest and NSubstitute
• Respect the project's folder structure and naming conventions

Related Issue

Closes #68

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 #78

---

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