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.App/Program.cs

@@ -0,0 +1,24 @@
+using Avalonia;
+using System;
+
+namespace ClaudeDo.App;
+
+sealed class Program
+{
+    // Initialization code. Don't use any Avalonia, third-party APIs or any
+    // SynchronizationContext-reliant code before AppMain is called: things aren't initialized
+    // yet and stuff might break.
+    [STAThread]
+    public static void Main(string[] args) => BuildAvaloniaApp()
+        .StartWithClassicDesktopLifetime(args);
+
+    // Avalonia configuration, don't remove; also used by visual designer.
+    public static AppBuilder BuildAvaloniaApp()
+        => AppBuilder.Configure<App>()
+            .UsePlatformDetect()
+#if DEBUG
+            .WithDeveloperTools()
+#endif
+            .WithInterFont()
+            .LogToTrace();
+}