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/10-performance-dedupe/repo/app/dedupe.py

@@ -0,0 +1,7 @@
+def dedupe(items):
+    """Return items without duplicates, keeping the first occurrence of each."""
+    result = []
+    for item in items:
+        if item not in result:
+            result.append(item)
+    return result