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.

tests/ClaudeDo.Ui.Tests/ViewModels/MergeHelperSelectionModalViewModelTests.cs

@@ -162,6 +162,105 @@ public class MergeHelperSelectionModalViewModelTests : IDisposable
         Assert.True(closed);
     }
 
+    [Fact]
+    public async Task Load_NestsChildUnderParent_InOrder()
+    {
+        await using (var db = NewContext())
+        {
+            db.Lists.Add(new ListEntity { Id = "L1", Name = "Work", CreatedAt = DateTime.UtcNow });
+            db.Tasks.Add(new TaskEntity { Id = "parent", ListId = "L1", Title = "parent", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 0 });
+            db.Tasks.Add(new TaskEntity { Id = "sibling", ListId = "L1", Title = "sibling", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 1 });
+            db.Tasks.Add(new TaskEntity { Id = "child", ListId = "L1", Title = "child", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 2, ParentTaskId = "parent" });
+            await db.SaveChangesAsync();
+        }
+
+        var vm = BuildVm();
+        vm.Configure("L1", "Work");
+        await vm.LoadAsync();
+
+        // Child is nested directly after its parent, ahead of the later sibling.
+        Assert.Equal(new[] { "parent", "child", "sibling" }, vm.Tasks.Select(t => t.Id));
+        Assert.True(vm.Tasks.Single(t => t.Id == "child").IsChild);
+        Assert.False(vm.Tasks.Single(t => t.Id == "child").IsOrphanChild);
+        Assert.False(vm.Tasks.Single(t => t.Id == "parent").IsChild);
+    }
+
+    [Fact]
+    public async Task Load_ChildOfFilteredOutParent_RendersTopLevelAsOrphan()
+    {
+        await using (var db = NewContext())
+        {
+            db.Lists.Add(new ListEntity { Id = "L1", Name = "Work", CreatedAt = DateTime.UtcNow });
+            db.Tasks.Add(new TaskEntity { Id = "parent-done", ListId = "L1", Title = "parent", Status = TaskStatus.Done, CreatedAt = DateTime.UtcNow, SortOrder = 0 });
+            db.Tasks.Add(new TaskEntity { Id = "child", ListId = "L1", Title = "child", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 1, ParentTaskId = "parent-done" });
+            await db.SaveChangesAsync();
+        }
+
+        var vm = BuildVm();
+        vm.Configure("L1", "Work");
+        await vm.LoadAsync();
+
+        // The parent is Done and filtered out; the child must not disappear, and must be
+        // flagged as an orphan instead of rendering as a (broken) child.
+        Assert.Single(vm.Tasks);
+        var row = vm.Tasks[0];
+        Assert.Equal("child", row.Id);
+        Assert.False(row.IsChild);
+        Assert.True(row.IsOrphanChild);
+    }
+
+    [Fact]
+    public async Task Toggle_Parent_CascadesSelectionToChildren()
+    {
+        await using (var db = NewContext())
+        {
+            db.Lists.Add(new ListEntity { Id = "L1", Name = "Work", CreatedAt = DateTime.UtcNow });
+            db.Tasks.Add(new TaskEntity { Id = "parent", ListId = "L1", Title = "parent", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 0 });
+            db.Tasks.Add(new TaskEntity { Id = "child1", ListId = "L1", Title = "child1", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 1, ParentTaskId = "parent" });
+            db.Tasks.Add(new TaskEntity { Id = "child2", ListId = "L1", Title = "child2", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 2, ParentTaskId = "parent" });
+            await db.SaveChangesAsync();
+        }
+
+        var vm = BuildVm();
+        vm.Configure("L1", "Work");
+        await vm.LoadAsync();
+        vm.SelectNoneCommand.Execute(null);
+        Assert.All(vm.Tasks, t => Assert.False(t.IsSelected));
+
+        vm.Tasks.Single(t => t.Id == "parent").IsSelected = true;
+        Assert.True(vm.Tasks.Single(t => t.Id == "child1").IsSelected);
+        Assert.True(vm.Tasks.Single(t => t.Id == "child2").IsSelected);
+
+        // A child can be individually deselected afterwards without touching the parent.
+        vm.Tasks.Single(t => t.Id == "child1").IsSelected = false;
+        Assert.True(vm.Tasks.Single(t => t.Id == "parent").IsSelected);
+        Assert.False(vm.Tasks.Single(t => t.Id == "child1").IsSelected);
+        Assert.True(vm.Tasks.Single(t => t.Id == "child2").IsSelected);
+
+        vm.Tasks.Single(t => t.Id == "parent").IsSelected = false;
+        Assert.False(vm.Tasks.Single(t => t.Id == "child2").IsSelected);
+    }
+
+    [Fact]
+    public async Task Confirm_ReturnsParentsBeforeChildren()
+    {
+        await using (var db = NewContext())
+        {
+            db.Lists.Add(new ListEntity { Id = "L1", Name = "Work", CreatedAt = DateTime.UtcNow });
+            db.Tasks.Add(new TaskEntity { Id = "parent", ListId = "L1", Title = "parent", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 0 });
+            db.Tasks.Add(new TaskEntity { Id = "child", ListId = "L1", Title = "child", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow, SortOrder = 1, ParentTaskId = "parent" });
+            await db.SaveChangesAsync();
+        }
+
+        var vm = BuildVm();
+        vm.Configure("L1", "Work");
+        await vm.LoadAsync();
+        vm.ConfirmCommand.Execute(null);
+
+        var result = await vm.Result.Task;
+        Assert.Equal(new[] { "parent", "child" }, result);
+    }
+
     [Fact]
     public async Task Load_NoCandidates_HasTasksFalse_CannotConfirm()
     {