feat(agent): systemd unit + release env.sh for root+journald install

agent/rel/proxmox-monitor-agent.service

@@ -0,0 +1,22 @@
+[Unit]
+Description=Proxmox Monitor Agent
+Documentation=https://github.com/you/proxmox_monitor
+After=network-online.target zfs.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=root
+Environment=AGENT_CONFIG=/etc/proxmox-monitor/agent.toml
+ExecStart=/usr/local/bin/proxmox-monitor-agent start
+ExecStop=/usr/local/bin/proxmox-monitor-agent stop
+Restart=always
+RestartSec=5
+# Burrito unpacks into this directory; keep it stable across runs
+Environment=BURRITO_CACHE_DIR=/var/cache/proxmox-monitor-agent
+
+# Resource limits
+LimitNOFILE=65536
+
+[Install]
+WantedBy=multi-user.target