feat(ui): show dequeue affordance on planning parents with queued children

Planning parents stay in Planning/Planned status while their children are
Queued/Waiting, so the existing IsQueued-only visibility rule hid the dequeue
button. Add HasQueuedSubtasks tracking and a CanRemoveFromQueue helper; the
parent-row dequeue cascades to all queued/waiting children. Also attach the
'agent' tag on explicit enqueue so the queue picker accepts the task.

src/ClaudeDo.Ui/Views/Islands/TaskRowView.axaml

@@ -35,7 +35,7 @@
                       IsVisible="{Binding !IsQueued}"
                       Click="OnSendToQueueClick"/>
             <MenuItem Header="Remove from queue"
-                      IsVisible="{Binding IsQueued}"
+                      IsVisible="{Binding CanRemoveFromQueue}"
                       Click="OnRemoveFromQueueClick"/>
             <Separator/>
             <MenuItem Header="Run interactively"
@@ -119,9 +119,9 @@
               <TextBlock Text="{Binding Status}"/>
             </Border>
 
-            <!-- Dequeue button (only when Queued) -->
+            <!-- Dequeue button (visible when row is Queued, or planning parent has queued subtasks) -->
             <Button Classes="icon-btn dequeue-btn"
-                    IsVisible="{Binding IsQueued}"
+                    IsVisible="{Binding CanRemoveFromQueue}"
                     ToolTip.Tip="Remove from queue"
                     Command="{Binding $parent[ItemsControl].((vm:TasksIslandViewModel)DataContext).RemoveFromQueueCommand}"
                     CommandParameter="{Binding}">