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/09-path-traversal/repo/app/files.py

@@ -0,0 +1,7 @@
+import os
+
+
+def read_upload(base, name):
+    """Return the bytes of the uploaded file name below the base directory."""
+    with open(os.path.join(base, name), "rb") as stream:
+        return stream.read()