Add GitHub Copilot agent hub for xAI .NET SDK

[Copilot]

Establishes specialized agents in .github/agents/ to provide context-aware assistance for different aspects of the codebase.

Agents

• dotnet-developer.md - .NET 8.0/10.0, C#, multi-targeting, NuGet packaging
• grpc-protocol.md - Protocol Buffers, gRPC code generation, proto file management
• testing.md - xUnit, Moq, test patterns and organization
• ai-extensions.md - Microsoft.Extensions.AI IChatClient implementation, AI tool abstractions (web search, X search, code execution, MCP)
• documentation.md - Technical writing, API docs, examples
• README.md - Agent hub index and quick reference

Each agent includes domain expertise, project-specific context (target frameworks, directory structure, proto files), and actionable guidelines.

Original prompt

agent hub

The user has attached the following file paths as relevant context:

• .github\copilot-instructions.md

Created from VS Code.

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