fix(worker): reuse shared hub fake and guard blank list name

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Worker/External/ListMcpTools.cs

@@ -47,6 +47,8 @@ public sealed class ListMcpTools
         var entity = await _lists.GetByIdAsync(listId, cancellationToken)
             ?? throw new InvalidOperationException($"List {listId} not found.");
 
+        if (name is not null && string.IsNullOrWhiteSpace(name))
+            throw new InvalidOperationException("name cannot be blank.");
         if (name is not null) entity.Name = name;
         if (workingDir is not null)
             entity.WorkingDir = string.IsNullOrWhiteSpace(workingDir) ? null : workingDir;