Add GitHub Copilot instructions file

[Copilot]

Adds .github/copilot-instructions.md per best practices guide at https://gh.io/copilot-coding-agent-tips.

Contents

• Project context: .NET 8 tool for scanning NuGet dependencies against license/version policies
• Tech stack: Nuke build system, MSTest, FluentAssertions, Verify snapshot testing, Spectre.Console
• Build/test patterns: Arrange-Act-Assert with real codebase examples
• Coding standards: C# Coding Guidelines enforcement via StyleCop, Roslynator, CSharpGuidelinesAnalyzer, Meziantou
• Contribution workflow: API approval process, using AcceptApiChanges.ps1/.sh for public API changes
• Common tasks: Adding license types, package sources, violation detection patterns
• Performance considerations: Caching behavior, GitHub rate limits for license fetching

Example test pattern documented

[TestMethod]
public async Task Can_deny_an_entire_package()
{
    // Arrange
    var analyzer = new CSharpProjectAnalyzer(cSharpProjectScanner, nuGetPackageAnalyzer)
    {
        ProjectPath = ProjectPath,
    };

    // Act
    var violations = await analyzer.ExecuteAnalysis(_ => new ProjectPolicy
    {
        DenyList = new DenyList { Packages = [new PackageSelector("FluentAssertions")] }
    });

    // Assert
    violations.Should().ContainEquivalentOf(new { PackageId = "FluentAssertions" });
}

Enables context-aware Copilot assistance for contributors.

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 ✨ Set up Copilot instructions #122

---

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