Add Copilot coding agent instructions

[Copilot]

Configures repository instructions for GitHub Copilot coding agent per best practices documentation.

Changes:

• Added .github/copilot-instructions.md with:
  • Build/test commands: dotnet restore, dotnet build, dotnet test with coverage settings
  • Project structure: core libraries, reference project, samples organization
  • Coding standards: .NET 8.0, C# 12.0, nullable types, strict analyzers
  • Critical patterns: structured logging (no string interpolation), Flurl v4 disposal, Rebus v8 exception handling
  • Key dependencies: NodaTime, SimpleInjector, Polly, Dapper, Reqnroll
  • Migration notes: v5.x breaking changes (Flurl, Rebus, .NET 8 minimum)
  • CI/CD: GitHub Actions workflows, CodeQL, NuGet audit

Example structured logging guidance:

// ❌ Avoid - evaluated even when logging disabled
_logger.Info($"Logon by {user} from {ip}");

// ✅ Use structured logging
_logger.Info(CultureInfo.InvariantCulture, "Logon by {user} from {ip}", user, ip);

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 #484

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