refactor(users): migrate system.user to system.users[] for multi-user support

Replace the single-user system.user dict with a system.users list to
support multiple user accounts. Update all roles, templates, examples,
validation, and documentation to use the new format. Remove redundant
post-normalization type checks from validation.yml.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

roles/global_defaults/tasks/main.yml

@@ -66,9 +66,9 @@
     - system_cfg.type == "virtual"
     - hypervisor_type != "vmware"
   ansible.builtin.set_fact:
-    ansible_user: "{{ system_cfg.user.name }}"
-    ansible_password: "{{ system_cfg.user.password }}"
-    ansible_become_password: "{{ system_cfg.user.password }}"
+    ansible_user: "{{ system_cfg.users[0].name }}"
+    ansible_password: "{{ system_cfg.users[0].password }}"
+    ansible_become_password: "{{ system_cfg.users[0].password }}"
     ansible_ssh_extra_args: "-o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no"
   changed_when: false