feat(14-01): add DirectoryDataGrid_MouseDoubleClick code-behind handler

- Extracts GraphDirectoryUser from DataGrid.SelectedItem on double-click
- Invokes SelectDirectoryUserCommand to add user to audit pipeline
- Using added for SharepointToolbox.Core.Models

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SharepointToolbox/Views/Tabs/UserAccessAuditView.xaml.cs

@@ -1,4 +1,5 @@
 using System.Windows.Controls;
+using SharepointToolbox.Core.Models;
 using SharepointToolbox.Services;
 using SharepointToolbox.ViewModels.Tabs;
 
@@ -24,4 +25,14 @@ public partial class UserAccessAuditView : UserControl
             listBox.SelectedItem = null;
         }
     }
+
+    private void DirectoryDataGrid_MouseDoubleClick(object sender, System.Windows.Input.MouseButtonEventArgs e)
+    {
+        if (sender is DataGrid grid && grid.SelectedItem is GraphDirectoryUser user)
+        {
+            var vm = (UserAccessAuditViewModel)DataContext;
+            if (vm.SelectDirectoryUserCommand.CanExecute(user))
+                vm.SelectDirectoryUserCommand.Execute(user);
+        }
+    }
 }