[Agent] Make the agent run as a systemd daemon

[FW-Nagorko]

Problem

Agent works for local dev but not as a proper system service: config is CWD-only (agent.conf), no install/unit story, mqueue control channel, no privilege model for RAPL in production.

Goal

Package and run voltad under systemd with FHS paths, a dedicated user, and a sane CLI (volta config init, control socket).

Done when

• cmake --install + systemctl enable --now volta-agent works
• Config from /etc/volta/agent.conf (overridable)
• Non-root user, RAPL still works
• README updated with install + smoke test"

Related

#18 - server already has runtime config; this issue covers the agent side