feat(14-01): add SelectDirectoryUserCommand bridging directory to audit pipeline

- RelayCommand<GraphDirectoryUser> converts to GraphUserResult and adds to SelectedUsers
- Duplicate UPN check prevents adding same user twice
- Initialized in both DI and test constructors
- 4 new tests pass (add, skip duplicate, null, auditable)

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

SharepointToolbox/ViewModels/Tabs/UserAccessAuditViewModel.cs

@@ -137,6 +137,7 @@ public partial class UserAccessAuditViewModel : FeatureViewModelBase
     public IAsyncRelayCommand ExportHtmlCommand { get; }
     public RelayCommand<GraphUserResult> AddUserCommand { get; }
     public RelayCommand<GraphUserResult> RemoveUserCommand { get; }
+    public RelayCommand<GraphDirectoryUser> SelectDirectoryUserCommand { get; }
     public IAsyncRelayCommand LoadDirectoryCommand { get; }
     public RelayCommand CancelDirectoryLoadCommand { get; }
 
@@ -174,6 +175,7 @@ public partial class UserAccessAuditViewModel : FeatureViewModelBase
         ExportHtmlCommand = new AsyncRelayCommand(ExportHtmlAsync, CanExport);
         AddUserCommand = new RelayCommand<GraphUserResult>(ExecuteAddUser);
         RemoveUserCommand = new RelayCommand<GraphUserResult>(ExecuteRemoveUser);
+        SelectDirectoryUserCommand = new RelayCommand<GraphDirectoryUser>(ExecuteSelectDirectoryUser);
 
         SelectedUsers.CollectionChanged += (_, _) => OnPropertyChanged(nameof(SelectedUsersLabel));
 
@@ -216,6 +218,7 @@ public partial class UserAccessAuditViewModel : FeatureViewModelBase
         ExportHtmlCommand = new AsyncRelayCommand(ExportHtmlAsync, CanExport);
         AddUserCommand = new RelayCommand<GraphUserResult>(ExecuteAddUser);
         RemoveUserCommand = new RelayCommand<GraphUserResult>(ExecuteRemoveUser);
+        SelectDirectoryUserCommand = new RelayCommand<GraphDirectoryUser>(ExecuteSelectDirectoryUser);
 
         SelectedUsers.CollectionChanged += (_, _) => OnPropertyChanged(nameof(SelectedUsersLabel));
 
@@ -548,6 +551,16 @@ public partial class UserAccessAuditViewModel : FeatureViewModelBase
         SelectedUsers.Remove(user);
     }
 
+    private void ExecuteSelectDirectoryUser(GraphDirectoryUser? dirUser)
+    {
+        if (dirUser == null) return;
+        var userResult = new GraphUserResult(dirUser.DisplayName, dirUser.UserPrincipalName, dirUser.Mail);
+        if (!SelectedUsers.Any(u => u.UserPrincipalName == userResult.UserPrincipalName))
+        {
+            SelectedUsers.Add(userResult);
+        }
+    }
+
     private async Task DebounceSearchAsync(string query, CancellationToken ct)
     {
         try