Set up Copilot instructions

[Copilot]

Description

Resolves #2323

Adds .github/copilot-instructions.md to configure GitHub Copilot coding agent per best practices.

Contents:

• Project overview and repository structure
• Build/test commands (dotnet build, dotnet test, dotnet pack)
• Multi-targeting guidance (netstandard2.0 through net10.0, conditional compilation)
• Code style: C# preview, nullable settings, license header template
• Testing: xUnit + FluentAssertions + AutoFixture, WireMock for HTTP
• Source generators: [GenerateImmutable], [GenerateClone], [Exclude]
• Central package management via Directory.Packages.props
• PR checklist

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

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 #2324

---

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

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