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/handlers/main.yml

@@ -0,0 +1,7 @@
+---
+- name: Enable and start check_mk socket
+  ansible.builtin.systemd:
+    name: check_mk.socket
+    enabled: true
+    state: started
+    daemon_reload: true