feat(review): gate Approve & Merge behind opening the diff

Approve & Merge is disabled until the pending changes have been inspected:
DetailsIslandViewModel tracks ReviewDiffViewed (reset on task switch and on
every state change), MergeSectionViewModel raises DiffViewed when a diff or
combined diff is opened and exposes HasReviewableDiff, and a hint sits next
to the button. A review with nothing to inspect (sandbox run, no worktree)
approves straight through.

ClaudeDo-Task: f9809a93

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

@@ -48,6 +48,15 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
     public Func<DiffViewerViewModel, System.Threading.Tasks.Task>? ShowDiffViewer { get; set; }
     public Func<MergeModalViewModel, System.Threading.Tasks.Task>? ShowMergeModal { get; set; }
 
+    // Raised when the user opens a diff/combined-diff for the current task, so the
+    // review gate can record that the changes were inspected before merging.
+    public Action? DiffViewed { get; set; }
+
+    // True when there is something to inspect before merging (a live worktree diff,
+    // a merged commit range, or a planning/children combined diff). When false there
+    // is nothing to read, so the review gate must not block approve.
+    public bool HasReviewableDiff => CanOpenDiff() || CanReviewDiff();
+
     public MergeSectionViewModel(IWorkerClient worker, IServiceProvider services)
     {
         _worker = worker;
@@ -69,6 +78,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
         _worktreeStateLabel = worktreeState;
         _listWorkingDir = listWorkDir;
         OnPropertyChanged(nameof(ShowMergeSection));
+        OnPropertyChanged(nameof(HasReviewableDiff));
         OpenDiffCommand.NotifyCanExecuteChanged();
         OpenWorktreeCommand.NotifyCanExecuteChanged();
     }
@@ -79,6 +89,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
         TaskTitle = taskTitle;
         _isPlanningParent = isPlanningParent;
         OnPropertyChanged(nameof(ShowMergeSection));
+        OnPropertyChanged(nameof(HasReviewableDiff));
     }
 
     internal void SyncChildOutcomes(bool hasChildOutcomes, int subtaskCount)
@@ -86,6 +97,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
         _hasChildOutcomes = hasChildOutcomes;
         _subtaskCount = subtaskCount;
         OnPropertyChanged(nameof(ShowMergeSection));
+        OnPropertyChanged(nameof(HasReviewableDiff));
         ReviewCombinedDiffCommand.NotifyCanExecuteChanged();
     }
 
@@ -128,6 +140,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
         var vm = _services.GetRequiredService<DiffViewerViewModel>();
         vm.ConfigurePlanning(TaskId, SelectedMergeTarget ?? "main");
         await vm.LoadAsync();
+        DiffViewed?.Invoke();
         await ShowDiffViewer(vm);
     }
 
@@ -157,6 +170,7 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
         else return;
 
         await vm.LoadAsync();
+        DiffViewed?.Invoke();
         await ShowDiffViewer(vm);
     }