Add practical C# / .NET integration guide for the Copilot SDK

[Copilot]

No step-by-step guide existed for C# developers integrating the Copilot SDK from scratch in Visual Studio or VS Code.

New: docs/guides/dotnet-integration.md

A 17-section, end-to-end walkthrough covering everything from blank project to production-ready patterns:

• Project setup — Visual Studio, VS Code + C# Dev Kit, and CLI flows
• NuGet installation and .csproj verification
• Minimal integration → multi-turn chat loop → streaming delta events
• Custom tools via AIFunctionFactory.Create
• File attachments, session hooks (pre/post-tool, prompt, lifecycle), infinite sessions + ResumeSessionAsync, and ask_user support
• IDE tips for Visual Studio 2022, VS Code C# Dev Kit, and Rider
• Common patterns: fire-and-collect, cancellation, parallel sessions
• Auth options: signed-in CLI, env vars, GitHubToken, BYOK
• Full worked example: streaming weather assistant with a custom tool

await using var client = new CopilotClient();
await using var session = await client.CreateSessionAsync(new SessionConfig
{
    Model = "gpt-4.1",
    Streaming = true,
    Tools =
    [
        AIFunctionFactory.Create(
            async ([Description("City name")] string city) =>
                await WeatherService.GetWeatherAsync(city),
            "get_weather",
            "Get current weather for a city")
    ],
    OnPermissionRequest = PermissionHandler.ApproveAll
});

Cross-references added

• docs/guides/setup/index.md — new Decision Matrix row + paragraph
• docs/getting-started.md — "Learn More" link
• README.md — "Quick Links" entry

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