feat(oam): add LibreNMS/check_mk monitoring agent role

Port Modules/OAM/oam-librenms.sh.

Deploys the check_mk agent and systemd socket for LibreNMS monitoring,
along with the local plugins: distro, dmi, dpkg, ntp-client,
os-updates, postfixdetailed, and postfix-queues.

🤖 Generated with [Crush](https://github.com/charmassociates/crush)

Assisted-by: GLM-5 via Crush <crush@charm.land>

roles/oam/files/librenms/postfix-queues

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+#Written by Valec 2006. Steal and share.
+#Get postfix queue lengths
+
+#extend mailq /opt/observer/scripts/getmailq.sh
+
+QUEUES="incoming active deferred hold"
+
+for i in $QUEUES; do
+        COUNT=$(qshape "$i" | grep TOTAL | awk '{print $2}')
+        printf "$COUNT\n"
+done