feat(details): attach clipboard image on Ctrl+V in the task description

Extracts the clipboard bitmap read into a shared ClipboardBitmap helper
(also used by the terminal view); the description TextBox now routes an
image paste through AddFilesAsync as a unique paste-*.png attachment.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/Detail/DescriptionStepsCard.axaml

@@ -103,7 +103,8 @@
           <!-- Description -->
           <Panel IsVisible="{Binding IsDescriptionSection}">
             <!-- Edit mode: raw TextBox -->
-            <TextBox Text="{Binding EditableDescription, Mode=TwoWay}"
+            <TextBox x:Name="DescriptionBox"
+                     Text="{Binding EditableDescription, Mode=TwoWay}"
                      AcceptsReturn="True"
                      TextWrapping="Wrap"
                      MinHeight="80"

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

@@ -3,6 +3,7 @@ using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.Platform.Storage;
+using ClaudeDo.Ui.Services;
 using ClaudeDo.Ui.ViewModels.Islands;
 
 namespace ClaudeDo.Ui.Views.Islands.Detail;
@@ -12,6 +13,41 @@ public partial class DescriptionStepsCard : UserControl
     public DescriptionStepsCard()
     {
         InitializeComponent();
+        // Tunnel: the TextBox handles Ctrl+V itself (text only), so an image paste has to be caught first.
+        DescriptionBox.AddHandler(KeyDownEvent, OnDescriptionPreviewKeyDown, RoutingStrategies.Tunnel);
+    }
+
+    private void OnDescriptionPreviewKeyDown(object? sender, KeyEventArgs e)
+    {
+        if (e.Key != Key.V || !e.KeyModifiers.HasFlag(KeyModifiers.Control) || e.KeyModifiers.HasFlag(KeyModifiers.Alt))
+            return;
+
+        // The clipboard read is async, so swallow the key and replay the text paste when there is no image.
+        e.Handled = true;
+        _ = PasteAsync();
+    }
+
+    /// <summary>Attaches a clipboard bitmap as a PNG file; anything else falls back to the normal text paste.</summary>
+    private async System.Threading.Tasks.Task PasteAsync()
+    {
+        try
+        {
+            using var bitmap = await ClipboardBitmap.TryGetAsync(TopLevel.GetTopLevel(this));
+            if (bitmap is null || DataContext is not DetailsIslandViewModel vm)
+            {
+                DescriptionBox.Paste();
+                return;
+            }
+
+            using var png = new System.IO.MemoryStream();
+            bitmap.Save(png);
+            png.Position = 0;
+            await vm.AddFilesAsync(new[] { (ClipboardImageTempStore.NewPngFileName(DateTime.UtcNow), (System.IO.Stream)png) });
+        }
+        catch (Exception)
+        {
+            // Paste is best-effort; a failed clipboard/encode must not surface as an unhandled UI exception.
+        }
     }
 
     private async void OnCopyClick(object? sender, RoutedEventArgs e)