Enable rootful Docker in the agent VM

[jimpudar]

Summary

• Enable rootful Docker in agent-vm.nix and start it at boot.
• Add the agent user to the docker group so containers are usable from the VM shell.
• Keep Docker from interfering with the Lima network proof by allowing only Docker bridge/veth interfaces there.
• Update docs and tests to reflect Docker as part of the agent VM contract.

Testing

• Added unit coverage for the Docker provisioning config and the adjusted Lima proof gate.
• Added integration assertions that Docker is active, docker compose is available, and a container smoke test can run inside the agent VM.
• Not run (not requested).