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.

tests/ClaudeDo.Ui.Tests/RepoScannerTests.cs

@@ -117,6 +117,29 @@ public sealed class RepoScannerTests : IDisposable
         Assert.Empty(result);
     }
 
+    [Fact]
+    public void Scan_ReturnsEmptyWhenRootIsAReparsePoint()
+    {
+        var repo = MakeDir("real-repo");
+        Directory.CreateDirectory(Path.Combine(repo, ".git"));
+
+        var junction = Path.Combine(_root, "junction-to-real-repo");
+        var mklink = System.Diagnostics.Process.Start(new System.Diagnostics.ProcessStartInfo
+        {
+            FileName = "cmd.exe",
+            Arguments = $"/c mklink /J \"{junction}\" \"{repo}\"",
+            UseShellExecute = false,
+            RedirectStandardOutput = true,
+            RedirectStandardError = true,
+        })!;
+        mklink.WaitForExit();
+        Assert.True(Directory.Exists(junction), "Failed to create test junction: " + mklink.StandardError.ReadToEnd());
+
+        var result = RepoScanner.Scan(junction);
+
+        Assert.Empty(result);
+    }
+
     [Fact]
 #pragma warning disable CA1416 // ACL manipulation is Windows-only; this test targets the Windows dev/CI environment
     public void Scan_ContinuesAfterUnreadableSubdirectory()