feat(ui): add merge-target dropdown and merge-all controls to planning detail

- Add SubtaskDiffDto and CombinedDiffResultDto to PlanningDtos.cs
- Extend IWorkerClient with 5 planning merge methods and 5 events
- Implement methods and hub subscriptions on WorkerClient
- Add Status and WorktreeState to SubtaskRowViewModel
- Add MergeTargetBranches, SelectedMergeTarget, CanMergeAll,
  MergeAllDisabledReason, MergeAllError, RecomputeCanMergeAll,
  MergeAllCommand, ReviewCombinedDiffCommand (Task 14 TODO)
  to DetailsIslandViewModel
- Add planning merge section to DetailsIslandView.axaml
  (merge target ComboBox + buttons + error label), gated on
  Task.IsPlanningParent
- Add 4 xUnit tests covering CanMergeAll logic and DTO shape

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

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml

@@ -138,6 +138,35 @@
     <ScrollViewer VerticalScrollBarVisibility="Auto">
       <StackPanel Spacing="0">
 
+        <!-- Planning merge section — visible only for planning parent tasks -->
+        <Border Padding="18,12,18,12"
+                BorderBrush="{DynamicResource LineBrush}"
+                BorderThickness="0,0,0,1"
+                IsVisible="{Binding Task.IsPlanningParent}">
+          <StackPanel Spacing="8">
+            <TextBlock Classes="section-label" Text="MERGE" Margin="0,0,0,2"/>
+            <StackPanel Spacing="4">
+              <TextBlock Text="Merge target"
+                         FontSize="11"
+                         Foreground="{DynamicResource TextFaintBrush}"/>
+              <ComboBox ItemsSource="{Binding MergeTargetBranches}"
+                        SelectedItem="{Binding SelectedMergeTarget, Mode=TwoWay}"
+                        HorizontalAlignment="Stretch"/>
+            </StackPanel>
+            <StackPanel Orientation="Horizontal" Spacing="8">
+              <Button Content="Review combined diff"
+                      Command="{Binding ReviewCombinedDiffCommand}"/>
+              <Button Content="Merge all subtasks"
+                      Command="{Binding MergeAllCommand}"
+                      ToolTip.Tip="{Binding MergeAllDisabledReason}"/>
+            </StackPanel>
+            <TextBlock Text="{Binding MergeAllError}"
+                       Foreground="OrangeRed"
+                       TextWrapping="Wrap"
+                       IsVisible="{Binding MergeAllError, Converter={x:Static ObjectConverters.IsNotNull}}"/>
+          </StackPanel>
+        </Border>
+
         <!-- Steps section -->
         <Border Padding="18,12,18,12"
                 BorderBrush="{DynamicResource LineBrush}"