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/03-delegation-parsers/repo/app/parsers/json_parser.py

@@ -0,0 +1,6 @@
+def parse_json_lines(text):
+    """Parse JSON Lines text into a list of objects.
+
+    Skip blank lines. Raise ValueError naming the 1-based line number for invalid JSON.
+    """
+    raise NotImplementedError