feat(planning): prevent orphaned subtasks via guards + startup repair

Three coordinated guards close the orphan-creation paths:

- CreateChildAsync refuses when the parent is not in a planning phase.
- DiscardPlanningAsync now returns a structured DiscardPlanningOutcome
  and refuses when children are queued or running; callers can opt into
  auto-dequeuing queued kids via dequeueQueuedChildren=true. Terminal
  children (Done/Failed/Cancelled) are promoted to top-level instead of
  becoming orphans when the parent's PlanningPhase is reset.
- OrphanRecovery hosted service clears ParentTaskId on any rows whose
  parent is missing or no longer in a planning phase on worker startup,
  mirroring the StaleTaskRecovery pattern.

UI surfaces the block reason: a confirm dialog offers to dequeue queued
children and retry; a running-children block is shown as a hard error
asking the user to cancel first.

WorkerClient now negotiates the JsonStringEnumConverter so the
DiscardPlanningResult enum round-trips correctly over SignalR.

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

src/ClaudeDo.Ui/Views/Islands/TasksIslandView.axaml.cs

@@ -2,6 +2,8 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Interactivity;
+using Avalonia.Layout;
+using Avalonia.Media;
 using Avalonia.VisualTree;
 using ClaudeDo.Ui.ViewModels.Islands;
 using ClaudeDo.Ui.ViewModels.Modals;
@@ -33,10 +35,55 @@ public partial class TasksIslandView : UserControl
                     await modal.ShowDialog(owner);
                     // ShowDialog completes once the window is closed (CloseAction or OS close).
                 };
+                vm.ConfirmAsync = ShowConfirmAsync;
             }
         };
     }
 
+    private async System.Threading.Tasks.Task<bool> ShowConfirmAsync(string message)
+    {
+        var owner = TopLevel.GetTopLevel(this) as Window;
+        if (owner is null) return false;
+
+        var tcs = new TaskCompletionSource<bool>();
+        var cancel  = new Button { Content = "Cancel",  MinWidth = 90 };
+        var confirm = new Button { Content = "Confirm", MinWidth = 90, Classes = { "danger" } };
+
+        var dialog = new Window
+        {
+            Title = "Confirm",
+            Width = 380,
+            SizeToContent = SizeToContent.Height,
+            CanResize = false,
+            WindowStartupLocation = WindowStartupLocation.CenterOwner,
+            ShowInTaskbar = false,
+            Background = this.FindResource("SurfaceBrush") as IBrush,
+            Content = new StackPanel
+            {
+                Margin  = new Thickness(20),
+                Spacing = 16,
+                Children =
+                {
+                    new TextBlock { Text = message, TextWrapping = TextWrapping.Wrap },
+                    new StackPanel
+                    {
+                        Orientation = Orientation.Horizontal,
+                        HorizontalAlignment = HorizontalAlignment.Right,
+                        Spacing = 8,
+                        Children = { cancel, confirm },
+                    },
+                },
+            },
+        };
+
+        cancel.Click  += (_, _) => { tcs.TrySetResult(false); dialog.Close(); };
+        confirm.Click += (_, _) => { tcs.TrySetResult(true);  dialog.Close(); };
+        dialog.Closed += (_, _) => tcs.TrySetResult(false);
+
+        _ = dialog.ShowDialog(owner);
+        return await tcs.Task;
+    }
+
     private async void OnTunnelPointerPressed(object? sender, PointerPressedEventArgs e)
     {
         if (DataContext is not TasksIslandViewModel vm) return;