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/11-search-expired-evaluation/task.json

@@ -0,0 +1,9 @@
+{
+  "prompt": "Customers whose free trial has ended can still download their monthly statements. Block that by raising PermissionError, and keep it working for paying customers and for customers whose trial is still running.",
+  "generate": {
+    "filler_modules": 250
+  },
+  "workflow": {
+    "kind": "concept search in a large repository"
+  }
+}