feat(ui): add MissionControl window + grid

src/ClaudeDo.Ui/Views/MissionControl/MissionControlWindow.axaml.cs

@@ -0,0 +1,17 @@
+using Avalonia.Controls;
+
+namespace ClaudeDo.Ui.Views.MissionControl;
+
+public partial class MissionControlWindow : Window
+{
+    public MissionControlWindow() => InitializeComponent();
+
+    protected override void OnClosing(WindowClosingEventArgs e)
+    {
+        // Hide instead of destroying — Mission Control keeps tracking tasks in the
+        // background and re-shows on next open.
+        e.Cancel = true;
+        Hide();
+        base.OnClosing(e);
+    }
+}