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/tasks/main.yml

@@ -0,0 +1,57 @@
+---
+# LibreNMS / check_mk agent — port of Modules/OAM/oam-librenms.sh.
+
+- name: Ensure /usr/lib/check_mk_agent tree exists
+  ansible.builtin.file:
+    path: "{{ item }}"
+    state: directory
+    owner: root
+    group: root
+    mode: "0755"
+  loop:
+    - /usr/lib/check_mk_agent
+    - /usr/lib/check_mk_agent/plugins
+    - /usr/lib/check_mk_agent/local
+
+- name: Deploy distro helper
+  ansible.builtin.copy:
+    src: librenms/distro
+    dest: /usr/local/bin/distro
+    owner: root
+    group: root
+    mode: "0755"
+
+- name: Deploy check_mk_agent
+  ansible.builtin.copy:
+    src: librenms/check_mk_agent
+    dest: /usr/bin/check_mk_agent
+    owner: root
+    group: root
+    mode: "0755"
+
+- name: Deploy check_mk systemd units
+  ansible.builtin.copy:
+    src: "librenms/{{ item.src }}"
+    dest: "/etc/systemd/system/{{ item.src }}"
+    owner: root
+    group: root
+    mode: "0644"
+  loop:
+    - { src: check_mk@.service }
+    - { src: check_mk.socket }
+  notify: Enable and start check_mk socket
+
+- name: Deploy check_mk local plugins
+  ansible.builtin.copy:
+    src: "librenms/{{ item }}"
+    dest: "/usr/lib/check_mk_agent/local/{{ item }}"
+    owner: root
+    group: root
+    mode: "0755"
+  loop:
+    - dmi.sh
+    - dpkg.sh
+    - ntp-client
+    - os-updates.sh
+    - postfixdetailed
+    - postfix-queues