chore(scaffold): add solution skeleton for App, Ui, Data, Worker and tests

Create .slnx with five projects, wire references (App->Ui->Data,
Worker->Data, Tests->Worker+Data), install Avalonia/MVVM/SignalR.Client/
Sqlite/Hosting packages, and add schema.sql per docs/plan.md.

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

src/ClaudeDo.Worker/Program.cs

@@ -0,0 +1,6 @@
+var builder = WebApplication.CreateBuilder(args);
+var app = builder.Build();
+
+app.MapGet("/", () => "Hello World!");
+
+app.Run();