fix(ui): reparse-check repo-scan root folder and move scan off UI thread

RepoScanner.Scan() only guarded reparse points on subdirectories found
during recursion, so a configured import root that is itself a junction
walked straight through onto other drives. Apply the same
FileAttributes.ReparsePoint check to the root before scanning.

RepoImportModalViewModel.ScanAndAdd ran the full 5-level recursive scan
synchronously on the UI thread; pointing the folder picker at a broad
directory froze the app. Offload RepoScanner.Scan to Task.Run per folder
and apply results back on the UI thread.

Add a RepoScannerTests case that creates a real junction (mklink /J) at
the scan root and asserts Scan returns empty.

src/ClaudeDo.Ui/ViewModels/Modals/RepoImportModalViewModel.cs

@@ -75,7 +75,7 @@ public sealed partial class RepoImportModalViewModel : ViewModelBase
             ErrorReported?.Invoke(Loc.T("vm.repoImport.loadFailed", ex.Message));
         }
 
-        ScanAndAdd(_folders);
+        await ScanAndAddAsync(_folders);
         OnPropertyChanged(nameof(HasFolders));
         NotifyCreateState();
     }
@@ -88,20 +88,21 @@ public sealed partial class RepoImportModalViewModel : ViewModelBase
 
         if (added.Count == 0) return;
 
-        ScanAndAdd(added);
+        await ScanAndAddAsync(added);
         OnPropertyChanged(nameof(HasFolders));
         NotifyCreateState();
         await SaveFoldersAsync();
     }
 
-    private void ScanAndAdd(IEnumerable<string> folders)
+    private async Task ScanAndAddAsync(IEnumerable<string> folders)
     {
         var current = new HashSet<string>(
             Repos.Select(r => r.FullPath), StringComparer.OrdinalIgnoreCase);
 
         foreach (var folder in folders)
         {
-            foreach (var item in BuildCandidates(RepoScanner.Scan(folder), current, _existingDirs))
+            var found = await Task.Run(() => RepoScanner.Scan(folder));
+            foreach (var item in BuildCandidates(found, current, _existingDirs))
             {
                 item.PropertyChanged += OnItemChanged;
                 Repos.Add(item);