implement session management in agent harness

agent-sessions/Dockerfile

@@ -0,0 +1,8 @@
+FROM python:3.12-slim
+
+# The agent's tool implementations (filesystem/shell/web) are bind-mounted
+# into /agent_tools at runtime, so the image only needs their dependencies.
+RUN pip install --no-cache-dir beautifulsoup4
+
+# Keep the container alive; the host runs tools via `docker exec`.
+CMD ["sleep", "infinity"]