fix(ui): block delete on interactive-session tasks, clear ghost selection

Delete (row context menu + detail header) now gates on HasInteractiveSession
the same way SendToQueue already does, so an open ConPTY session's claude
process can't be orphaned by deleting its task out from under it.

Row removal (in-UI delete and the delta-refresh path for externally deleted
tasks) now clears SelectedTask via SelectFrom(null, "row-removed") when the
removed row was selected, so the detail pane no longer keeps showing a
deleted task with live buttons.

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

@@ -110,6 +110,21 @@ public class DetailsIslandDeleteTaskTests : IDisposable
         Assert.False(vm.DeleteTaskCommand.CanExecute(null));
     }
 
+    [Fact]
+    public void DeleteTask_WhenTaskHasInteractiveSession_CommandIsDisabled()
+    {
+        var worker = new RecordingWorkerClient();
+        var vm = BuildVm(worker);
+        var row = new TaskRowViewModel { Id = "task-del-5", Status = TaskStatus.Idle };
+        vm.Bind(row);
+
+        Assert.True(vm.DeleteTaskCommand.CanExecute(null));
+
+        row.HasInteractiveSession = true;
+
+        Assert.False(vm.DeleteTaskCommand.CanExecute(null));
+    }
+
     [Fact]
     public async Task DeleteTask_WhenWorkerThrowsInvalidOperationException_SurfacesErrorAndKeepsDetailOpen()
     {