Add GitHub Copilot instructions for repository conventions and workflows

[Copilot]

Configures GitHub Copilot coding agent with repository-specific context per best practices.

Changes

• .github/copilot-instructions.md - Repository-wide instructions covering:
  • Build/test/format commands (dotnet build, dotnet test, dotnet format)
  • Project structure (Libraries/, Samples/, Tests/ organization)
  • C# conventions from .editorconfig (naming: PascalCase/camelCase/_camelCase, file-scoped namespaces, explicit types)
  • Testing patterns and test project naming
  • Security, performance, and maintainability guidelines
  • Common workflows (adding libraries, samples, API changes)
  • CI/CD workflow details

This enables Copilot to provide context-aware suggestions aligned with Teams.NET SDK conventions without requiring manual guidance.

Original prompt

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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 #251

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