feat(01-01): create three-project .NET 9 solution scaffold

- ChatAgent.sln at repo root with Client, Api, Shared projects
- Blazor WASM client, ASP.NET Core Web API, shared class library
- Both Client and Api reference Shared project
- Removed template boilerplate (WeatherForecast, Class1)
- Added .NET ignores to .gitignore
- Pinned .NET 9 SDK via global.json

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ChatAgent.Client/Program.cs

@@ -0,0 +1,11 @@
+using Microsoft.AspNetCore.Components.Web;
+using Microsoft.AspNetCore.Components.WebAssembly.Hosting;
+using ChatAgent.Client;
+
+var builder = WebAssemblyHostBuilder.CreateDefault(args);
+builder.RootComponents.Add<App>("#app");
+builder.RootComponents.Add<HeadOutlet>("head::after");
+
+builder.Services.AddScoped(sp => new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });
+
+await builder.Build().RunAsync();