fix(ci): declare psutil runtime dependency

CI on main broke after PR #10 merge because agent_bench/serving/app.py
imports psutil unconditionally (used at line 111 for multi-corpus RSS
baseline logging) but psutil was never added to pyproject.toml
dependencies. Local test runs passed because psutil was installed in
the project's Python site-packages from some earlier ad-hoc install;
CI's fresh `pip install -e ".[dev]"` did not pick it up, producing
ModuleNotFoundError at test collection for three test modules that
import agent_bench.serving.app.

psutil.Process() and memory_info() are stable since psutil 5.x, so
`>=5.9.0` is a safe floor matching the style of the other declared
runtime deps.

Verified with full `pytest tests/` — 443 passed (same count as before
the CI break), no regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -18,6 +18,7 @@ dependencies = [
     "httpx>=0.27.0",
     "simpleeval>=1.0.0",
     "numpy>=1.26.0",
+    "psutil>=5.9.0",
     "langchain>=0.2.0,<1.0.0",
     "langchain-openai>=0.1.0",
     "langchain-anthropic>=0.1.0",