test(benchmarks): add A/B task fixtures with hidden checks

Ten small Python tasks, each failing on the fixture and passing with a
reference solution, including workflow expectations for a small change, a
bugfix, and a delegable change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/benchmarks/tasks/02-bugfix-pagination/repo/app/pagination.py

@@ -0,0 +1,5 @@
+def paginate(items, page, size):
+    """Return the 1-based page of items with the given page size."""
+    start = (page - 1) * size
+    end = min(start + size, len(items) - 1)
+    return items[start:end]