RouterBase .NET Client 0.1.0

Initial .NET client release for RouterBase.\n\nIncludes:\n- NuGet-ready package metadata for RouterBase.Client.\n- .NET 8 client for chat completions and model listing.\n- No external runtime dependencies.\n- Console-based test project with fake HTTP handler.\n- GitHub Actions CI covering restore, build, tests, and pack.\n\nDocs: https://docs.routerbase.com/