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/dpkg.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# Cache the file for 30 minutes
+# If you want to override this, put the command in cron.
+# We cache because it is a 1sec delay, which is painful for the poller
+if [ -x /usr/bin/dpkg-query ]; then
+  DATE=$(date +%s)
+  FILE=/var/cache/librenms/agent-local-dpkg
+  
+  [ -d /var/cache/librenms ] || mkdir -p /var/cache/librenms
+  
+  if [ ! -e $FILE ]; then
+    dpkg-query -W --showformat='${Status} ${Package} ${Version} ${Architecture} ${Installed-Size}\n'|grep " installed "|cut -d\  -f4- > $FILE
+  fi
+  FILEMTIME=$(stat -c %Y $FILE)
+  FILEAGE=$(($DATE-$FILEMTIME))
+  if [ $FILEAGE -gt 1800 ]; then
+    dpkg-query -W --showformat='${Status} ${Package} ${Version} ${Architecture} ${Installed-Size}\n'|grep " installed "|cut -d\  -f4- > $FILE
+  fi
+  echo "<<<dpkg>>>"
+  cat $FILE
+fi
+