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/task.json

@@ -0,0 +1,9 @@
+{
+  "prompt": "Uploads that fail with a temporary network error are never retried. Make at most three attempts in total before giving up with the last error; other errors must not be retried.",
+  "generate": {
+    "filler_modules": 250
+  },
+  "workflow": {
+    "kind": "concept search in a large repository"
+  }
+}