feat(ui): "Worktree öffnen" im Kontextmenü der Task-Zeile

Group 1 of the task-row context menu gets a fourth gated entry, right
after "Open interactive session": it opens the task's worktree folder
via Process.Start/UseShellExecute, mirroring ListsIslandViewModel's
OpenInExplorer. Missing worktree greys the item out with a tooltip
reason instead of hiding it, keeping Group 1's fixed length. Failures
report through TasksIslandViewModel.ErrorReported (footer strip), not
a silent catch.

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

@@ -61,6 +61,7 @@ public partial class TaskRowView : UserControl
         menu.Items.Add(removeFromQueueItem);
         menu.Items.Add(MakeItem("tasks.ctxCancelExecution", OnCancelExecutionClick, reason: row.CancelDisabledReason));
         menu.Items.Add(MakeItem("tasks.ctxOpenConPtySession", OnOpenConPtySessionClick, reason: row.QuickSessionDisabledReason));
+        menu.Items.Add(MakeItem("tasks.ctxOpenWorktree", OnOpenWorktreeClick, reason: row.OpenWorktreeDisabledReason));
         menu.Items.Add(MakeItem("tasks.ctxRefineTask", OnRefineTaskClick, reason: row.RefineDisabledReason));
         menu.Items.Add(new Separator());
 
@@ -169,6 +170,12 @@ public partial class TaskRowView : UserControl
             vm.OpenConPtySessionCommand.Execute(row);
     }
 
+    private void OnOpenWorktreeClick(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is TaskRowViewModel row && FindTasksVm() is { } vm)
+            vm.OpenTaskWorktreeCommand.Execute(row);
+    }
+
     // The status chip is only actionable while a ConPTY session is open; the open command dedupes
     // by task id, so it surfaces Mission Control and focuses the existing pane.
     private void OnStatusChipTapped(object? sender, RoutedEventArgs e)