Initial commit: Elixir MQTT system stats daemon

- MQTT client with configurable broker connection
- Periodic system stats publishing (30s interval)
- Command listening on MQTT topic with logging
- Systemd service configuration
- NixOS module for declarative deployment

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

system_stats_daemon.service

@@ -0,0 +1,26 @@
+[Unit]
+Description=System Stats MQTT Daemon
+After=network.target
+
+[Service]
+Type=exec
+User=root
+Group=root
+ExecStart=/opt/system_stats_daemon/bin/system_stats_daemon start
+ExecStop=/opt/system_stats_daemon/bin/system_stats_daemon stop
+Restart=always
+RestartSec=5
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=system_stats_daemon
+WorkingDirectory=/opt/system_stats_daemon
+
+# Security settings - still apply restrictions where possible
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectHome=true
+# Don't use ProtectSystem=strict since we need to read system stats
+ProtectSystem=false
+
+[Install]
+WantedBy=multi-user.target