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/ViewModels/Islands/TasksIslandViewModel.cs

@@ -1429,6 +1429,21 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         OpenConPtySessionRequested?.Invoke(row.Id);
     }
 
+    [RelayCommand]
+    private void OpenTaskWorktree(TaskRowViewModel? row)
+    {
+        if (row?.WorktreePath is not { } path || string.IsNullOrWhiteSpace(path)) return;
+        try
+        {
+            System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo
+            {
+                FileName = path,
+                UseShellExecute = true,
+            });
+        }
+        catch (Exception ex) { ErrorReported?.Invoke(Loc.T("vm.tasksIsland.openWorktreeFailed", ex.Message)); }
+    }
+
     [RelayCommand]
     private async Task ResumePlanningSessionAsync(TaskRowViewModel? row)
     {