feat(ui): show child tasks as children in the merge-helper picker

MergeHelperSelectionModalViewModel now loads ParentTaskId and orders
rows hierarchy-aware (parent immediately followed by its children,
mirroring TasksIslandViewModel.Regroup). Children render indented with
the same indent-track visual as the main task list; ticking a parent
cascades selection to its children, while a child can still be
deselected on its own. A child whose parent got filtered out (e.g.
already Done) renders top-level with an explicit hint instead of
disappearing. Confirm keeps returning parents ahead of their children.

src/ClaudeDo.Ui/ViewModels/Modals/MergeHelperSelectionModalViewModel.cs

@@ -14,6 +14,16 @@ public sealed partial class MergeHelperTaskRowViewModel : ViewModelBase
     public required string Id { get; init; }
     public required string Title { get; init; }
     public required string StatusText { get; init; }
+    public string? ParentTaskId { get; init; }
+
+    /// <summary>True when the parent is also a candidate and rendered directly above this row.</summary>
+    public bool IsChild { get; init; }
+
+    /// <summary>
+    /// True when <see cref="ParentTaskId"/> is set but the parent isn't a candidate itself
+    /// (e.g. already Done/Cancelled) — the row still renders top-level, flagged with a hint.
+    /// </summary>
+    public bool IsOrphanChild { get; init; }
 
     [ObservableProperty] private bool _isSelected;
 }
@@ -28,6 +38,9 @@ public sealed partial class MergeHelperSelectionModalViewModel : ViewModelBase
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
     private string _listId = "";
 
+    // Parent id -> its child rows, built after load; drives the tick-parent-ticks-children cascade.
+    private Dictionary<string, List<MergeHelperTaskRowViewModel>> _childrenByParentId = new();
+
     public ObservableCollection<MergeHelperTaskRowViewModel> Tasks { get; } = new();
 
     [ObservableProperty] private string _scopeLabel = "";
@@ -51,6 +64,7 @@ public sealed partial class MergeHelperSelectionModalViewModel : ViewModelBase
     {
         foreach (var row in Tasks) row.PropertyChanged -= OnRowChanged;
         Tasks.Clear();
+        _childrenByParentId = new();
 
         await using var ctx = await _dbFactory.CreateDbContextAsync(ct);
         var candidates = await ctx.Tasks.AsNoTracking()
@@ -59,21 +73,56 @@ public sealed partial class MergeHelperSelectionModalViewModel : ViewModelBase
             // Manual tasks are reminders the user owns — never offer them to the handler.
             .Where(t => !t.IsManual)
             .OrderBy(t => t.SortOrder).ThenBy(t => t.CreatedAt)
-            .Select(t => new { t.Id, t.Title, t.Status })
+            .Select(t => new { t.Id, t.Title, t.Status, t.ParentTaskId })
             .ToListAsync(ct);
 
-        foreach (var c in candidates)
+        var candidateIds = candidates.Select(c => c.Id).ToHashSet();
+        bool ParentIsCandidate(string? parentId) =>
+            !string.IsNullOrEmpty(parentId) && candidateIds.Contains(parentId);
+
+        var childrenByParent = candidates
+            .Where(c => ParentIsCandidate(c.ParentTaskId))
+            .GroupBy(c => c.ParentTaskId!)
+            .ToDictionary(g => g.Key, g => g.ToList());
+
+        // Hierarchy-ordered walk, same idea as TasksIslandViewModel.Regroup: top-level rows
+        // (including orphaned children whose parent isn't a candidate) interleaved with their
+        // visible children, preserving the SortOrder/CreatedAt order within each level.
+        var emitted = new HashSet<string>();
+        var ordered = new List<(string Id, string Title, TaskStatus Status, string? ParentTaskId)>();
+        void Emit(string id, string title, TaskStatus status, string? parentTaskId)
         {
+            if (!emitted.Add(id)) return;
+            ordered.Add((id, title, status, parentTaskId));
+            if (childrenByParent.TryGetValue(id, out var kids))
+                foreach (var kid in kids)
+                    Emit(kid.Id, kid.Title, kid.Status, kid.ParentTaskId);
+        }
+        foreach (var c in candidates.Where(c => !ParentIsCandidate(c.ParentTaskId)))
+            Emit(c.Id, c.Title, c.Status, c.ParentTaskId);
+
+        foreach (var c in ordered)
+        {
+            var isChild = ParentIsCandidate(c.ParentTaskId);
             var row = new MergeHelperTaskRowViewModel
             {
                 Id = c.Id,
                 Title = c.Title,
                 StatusText = c.Status.ToString(),
+                ParentTaskId = c.ParentTaskId,
+                IsChild = isChild,
+                IsOrphanChild = !isChild && !string.IsNullOrEmpty(c.ParentTaskId),
                 IsSelected = IsTickedByDefault(c.Status),
             };
             row.PropertyChanged += OnRowChanged;
             Tasks.Add(row);
         }
+
+        _childrenByParentId = Tasks
+            .Where(r => r.IsChild)
+            .GroupBy(r => r.ParentTaskId!)
+            .ToDictionary(g => g.Key, g => g.ToList());
+
         OnPropertyChanged(nameof(HasTasks));
         OnPropertyChanged(nameof(CanConfirm));
     }
@@ -85,8 +134,18 @@ public sealed partial class MergeHelperSelectionModalViewModel : ViewModelBase
 
     private void OnRowChanged(object? sender, PropertyChangedEventArgs e)
     {
-        if (e.PropertyName == nameof(MergeHelperTaskRowViewModel.IsSelected))
-            OnPropertyChanged(nameof(CanConfirm));
+        if (e.PropertyName != nameof(MergeHelperTaskRowViewModel.IsSelected)) return;
+
+        // Ticking/unticking a parent cascades to its children; a child can still be
+        // deselected on its own afterwards without affecting the parent's checkbox —
+        // ClaudeDo has no indeterminate/tri-state parent checkbox.
+        if (sender is MergeHelperTaskRowViewModel row
+            && _childrenByParentId.TryGetValue(row.Id, out var children))
+        {
+            foreach (var child in children) child.IsSelected = row.IsSelected;
+        }
+
+        OnPropertyChanged(nameof(CanConfirm));
     }
 
     [RelayCommand]