test(benchmarks): add concept-search tasks

Three tasks whose target code shares no vocabulary with the prompt, run in a
repository with generated distractor modules.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/benchmarks/tasks/12-search-transient-retry/repo/app/transport/courier.py

@@ -0,0 +1,3 @@
+def dispatch(blob, channel):
+    """Hand a blob to the channel and return the receipt."""
+    return channel.send(blob)