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/04-security-sql/task.json

@@ -0,0 +1,6 @@
+{
+  "prompt": "find_user() fails for user names that contain an apostrophe, such as O'Brien. Fix it.",
+  "workflow": {
+    "kind": "security-relevant bugfix"
+  }
+}