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/07-preserve-unknown-settings/repo/app/settings_store.py

@@ -0,0 +1,15 @@
+import json
+
+KNOWN = ("theme", "language")
+
+
+def save_settings(path, updates):
+    """Apply updates to the JSON settings file at path."""
+    try:
+        with open(path, encoding="utf-8") as stream:
+            current = json.load(stream)
+    except FileNotFoundError:
+        current = {}
+    merged = {key: updates.get(key, current.get(key)) for key in KNOWN if key in updates or key in current}
+    with open(path, "w", encoding="utf-8") as stream:
+        json.dump(merged, stream, indent=2)