feat(ui): add aggregated diff viewer for planning tasks

Implements Task 14: PlanningDiffView (Window), PlanningDiffViewModel,
ShowPlanningDiffModal callback wired in DetailsIslandView, and 5 xUnit tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -148,6 +148,9 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase
     // Set by the view so DeleteTaskCommand can prompt yes/no before deleting
     public Func<string, System.Threading.Tasks.Task<bool>>? ConfirmAsync { get; set; }
 
+    // Set by the view so ReviewCombinedDiffCommand can show the planning diff modal
+    public Func<ViewModels.Planning.PlanningDiffViewModel, System.Threading.Tasks.Task>? ShowPlanningDiffModal { get; set; }
+
     // Set by the view so DeleteTaskCommand can show an error message
     public Func<string, System.Threading.Tasks.Task>? ShowErrorAsync { get; set; }
 
@@ -570,8 +573,10 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase
     [RelayCommand(CanExecute = nameof(CanReviewDiff))]
     private async System.Threading.Tasks.Task ReviewCombinedDiffAsync()
     {
-        // TODO(Task 14): open PlanningDiffView once it exists
-        await System.Threading.Tasks.Task.CompletedTask;
+        if (Task is null || ShowPlanningDiffModal is null) return;
+        var vm = new ViewModels.Planning.PlanningDiffViewModel(_worker, Task.Id, SelectedMergeTarget ?? "main");
+        await vm.InitializeAsync();
+        await ShowPlanningDiffModal(vm);
     }
 
     private bool CanReviewDiff() => Task?.IsPlanningParent == true && Subtasks.Any();

src/ClaudeDo.Ui/ViewModels/Planning/PlanningDiffViewModel.cs

@@ -0,0 +1,91 @@
+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+using ClaudeDo.Ui.Services;
+
+namespace ClaudeDo.Ui.ViewModels.Planning;
+
+public sealed partial class PlanningDiffViewModel : ObservableObject
+{
+    private readonly IWorkerClient _worker;
+    private readonly string _planningTaskId;
+    private readonly string _targetBranch;
+
+    public ObservableCollection<SubtaskDiffRow> Subtasks { get; } = new();
+
+    [ObservableProperty] private SubtaskDiffRow? _selectedSubtask;
+    [ObservableProperty] private string _displayedDiff = "";
+    [ObservableProperty] private bool _isCombinedMode;
+    [ObservableProperty] private string? _combinedWarning;
+    [ObservableProperty] private bool _isLoadingCombined;
+
+    public Action? CloseAction { get; set; }
+
+    public PlanningDiffViewModel(IWorkerClient worker, string planningTaskId, string targetBranch)
+    {
+        _worker = worker;
+        _planningTaskId = planningTaskId;
+        _targetBranch = targetBranch;
+    }
+
+    [RelayCommand]
+    private void Close() => CloseAction?.Invoke();
+
+    public async Task InitializeAsync()
+    {
+        var items = await _worker.GetPlanningAggregateAsync(_planningTaskId);
+        Subtasks.Clear();
+        foreach (var i in items)
+            Subtasks.Add(new SubtaskDiffRow(i.SubtaskId, i.Title, i.DiffStat, i.UnifiedDiff));
+        SelectedSubtask = Subtasks.FirstOrDefault();
+    }
+
+    partial void OnSelectedSubtaskChanged(SubtaskDiffRow? value)
+    {
+        if (!IsCombinedMode)
+            DisplayedDiff = value?.UnifiedDiff ?? "";
+    }
+
+    [RelayCommand]
+    private async Task ToggleCombinedAsync()
+    {
+        if (IsCombinedMode)
+        {
+            IsLoadingCombined = true;
+            try
+            {
+                var result = await _worker.BuildPlanningIntegrationBranchAsync(_planningTaskId, _targetBranch);
+
+                if (result is null)
+                {
+                    DisplayedDiff = "";
+                    CombinedWarning = "Could not build combined preview (hub error).";
+                }
+                else if (result.Success)
+                {
+                    DisplayedDiff = result.UnifiedDiff ?? "";
+                    CombinedWarning = null;
+                }
+                else
+                {
+                    var files = result.ConflictedFiles?.Count ?? 0;
+                    CombinedWarning = $"Cannot build combined preview: subtask {result.FirstConflictSubtaskId} conflicts with an earlier subtask ({files} files).";
+                    DisplayedDiff = "";
+                }
+            }
+            finally
+            {
+                IsLoadingCombined = false;
+            }
+        }
+        else
+        {
+            DisplayedDiff = SelectedSubtask?.UnifiedDiff ?? "";
+            CombinedWarning = null;
+        }
+    }
+
+    partial void OnIsCombinedModeChanged(bool value) => ToggleCombinedCommand.Execute(null);
+}
+
+public sealed record SubtaskDiffRow(string Id, string Title, string? DiffStat, string UnifiedDiff);