feat(ui): add TaskHeaderBar detail component

Standalone UserControl for the task detail island redesign.
Grid layout: id badge + editable title | trash/skull toggle | gear flyout.
Skull geometry added to IslandStyles.axaml (Icon.Skull, EvenOdd fill).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/Detail/TaskHeaderBar.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace ClaudeDo.Ui.Views.Islands.Detail;
+
+public partial class TaskHeaderBar : UserControl
+{
+    public TaskHeaderBar()
+    {
+        InitializeComponent();
+    }
+}