refactor(ui): dedupe path-open helper across five view models

ListsIslandViewModel.OpenInExplorer, MergeSectionViewModel.OpenWorktree,
WorktreesOverviewModalViewModel.OpenInExplorer, AboutModalViewModel.OpenPath and
TasksIslandViewModel.OpenTaskWorktree each reimplemented "open this path in the
shell" with their own existence check, launch mechanism and error handling.
Replace all five with the new ShellOpen.Path helper and route failures through
the existing ErrorReported -> footer-strip convention instead of bare catch {}.

src/ClaudeDo.Ui/ViewModels/Islands/MergeSectionViewModel.cs

@@ -60,6 +60,10 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
     // review gate can record that the changes were inspected before merging.
     public Action? DiffViewed { get; set; }
 
+    // Mirrors ListsIslandViewModel/TasksIslandViewModel.ErrorReported — DetailsIslandViewModel
+    // forwards this into its own ErrorReported, which the shell flashes in the footer strip.
+    public event Action<string>? ErrorReported;
+
     // True when there is something to inspect before merging (a live worktree diff,
     // a merged commit range, or a planning/children combined diff). When false there
     // is nothing to read, so the review gate must not block approve.
@@ -191,16 +195,9 @@ public sealed partial class MergeSectionViewModel : ViewModelBase
     [RelayCommand(CanExecute = nameof(CanOpenWorktree))]
     private void OpenWorktree()
     {
-        if (_worktreePath is null) return;
-        try
-        {
-            System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo
-            {
-                FileName = _worktreePath,
-                UseShellExecute = true,
-            });
-        }
-        catch { }
+        var (ok, error) = ShellOpen.Path(_worktreePath);
+        if (!ok && error is not null)
+            ErrorReported?.Invoke(Loc.T("vm.mergeSection.openWorktreeFailed", error));
     }
 
     private bool CanOpenWorktree() => _worktreePath != null;