Ship systemd unit and maintainer scripts in .deb

[ChrisonSimtian]

Parent: #123

The .deb (L1) installs a systemd service unit and uses Debian maintainer scripts to register it without leaving the operator to copy files around.

Definition of done

• Unit installed at /lib/systemd/system/fallout-agent.service (system-level, not user).
• Unit Type=simple (or notify if the agent supports sd_notify — decide during implementation), Restart=on-failure, RestartSec=10s.
• Runs as a dedicated fallout-agent system user (created in postinst via adduser --system).
• postinst runs systemctl daemon-reload and systemctl enable fallout-agent (does NOT start — operator starts after config).
• prerm runs systemctl stop fallout-agent and systemctl disable fallout-agent.
• postrm purges /etc/fallout-agent/ only on purge, not on remove.
• Reinstall over an existing install does NOT double-enable or wipe /etc/fallout-agent/.