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.Ui/Views/MainWindow.axaml

@@ -0,0 +1,20 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:ClaudeDo.Ui.ViewModels"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+        x:Class="ClaudeDo.Ui.Views.MainWindow"
+        x:DataType="vm:MainWindowViewModel"
+        Icon="/Assets/avalonia-logo.ico"
+        Title="ClaudeDo.App">
+
+    <Design.DataContext>
+        <!-- This only sets the DataContext for the previewer in an IDE,
+             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
+        <vm:MainWindowViewModel/>
+    </Design.DataContext>
+
+    <TextBlock Text="{Binding Greeting}" HorizontalAlignment="Center" VerticalAlignment="Center"/>
+
+</Window>