Create Debian package for beszel-agent (#497)

* Add Debian package to build

Signed-off-by: Daniel Lo Nigro <git@d.sb>

* Fix Lintian issues

Signed-off-by: Daniel Lo Nigro <git@d.sb>

* - Add prerm script to stop and disable service
- Add security/sandboxing options to systemd unit

Signed-off-by: Daniel Lo Nigro <git@d.sb>

---------

Signed-off-by: Daniel Lo Nigro <git@d.sb>

supplemental/debian/beszel-agent.service

@@ -0,0 +1,32 @@
+[Unit]
+Description=Beszel Agent Service
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Environment="PORT=45876"
+# Port number can be overridden in beszel-agent.conf if needed
+EnvironmentFile=/etc/beszel-agent.conf
+ExecStart=/usr/bin/beszel-agent
+User=beszel
+Restart=on-failure
+StateDirectory=beszel-agent
+
+# Security/sandboxing settings
+KeyringMode=private
+LockPersonality=yes
+NoNewPrivileges=yes
+PrivateTmp=yes
+ProtectClock=yes
+ProtectHome=read-only
+ProtectHostname=yes
+ProtectKernel=yes
+ProtectKernelLogs=yes
+ProtectKernelTunables=yes
+ProtectSystem=strict
+RemoveIPC=yes
+RestrictSUIDSGID=true
+SystemCallArchitectures=native
+
+[Install]
+WantedBy=multi-user.target