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/05-write-tests/repo/app/slug.py

@@ -0,0 +1,9 @@
+import re
+import unicodedata
+
+
+def slugify(text, max_length=50):
+    """Lowercase ASCII slug with single hyphens, trimmed to max_length without a trailing hyphen."""
+    text = unicodedata.normalize("NFKD", text).encode("ascii", "ignore").decode()
+    text = re.sub(r"[^a-zA-Z0-9]+", "-", text).strip("-").lower()
+    return text[:max_length].rstrip("-")