README: mention agent/ in the layout

README.md

@@ -22,6 +22,7 @@ tasks/<family>/<task>/
     environment/        Dockerfile, reference.py, the stub the agent edits, measure.py
     tests/              the grader (verify_env.py) + test.sh
 tools/                  the generators that produce these tasks
+agent/                  a kernel-optimization skill + subagents for solving these tasks
 ```
 
 ## Running one task