fix(test): name the cancellationToken arg in the preview_merge_set subset test

Two branches merged into main independently: one added `runVerify` to
PreviewMergeSet ahead of `cancellationToken`, the other added the subset
test that passed the token positionally. Neither conflicted textually, so
the mismatch only surfaced as CS1503 at test-compile time.

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -2197,7 +2197,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         }
 
         var sut = BuildSut(CreateQueue());
-        var result = await sut.PreviewMergeSet([subsetTask.Id, supersetTask.Id], "main", CancellationToken.None);
+        var result = await sut.PreviewMergeSet([subsetTask.Id, supersetTask.Id], "main", cancellationToken: CancellationToken.None);
 
         var relation = Assert.Single(result.Subsets);
         Assert.Equal(subsetTask.Id, relation.TaskId);