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/validation.yml

@@ -48,25 +48,9 @@
     fail_msg: "Unsupported system keys: {{ system_unknown_keys | join(', ') }}."
     quiet: true
 
-- name: Validate nested system mappings
-  loop:
-    - network
-    - user
-    - root
-    - luks
-    - features
-  loop_control:
-    label: "{{ item }}"
-  ansible.builtin.assert:
-    that:
-      - system[item] is not defined or system[item] is mapping
-    fail_msg: "system.{{ item }} must be a dictionary."
-    quiet: true
-
 - name: Validate system sub-dict schemas
   loop:
     - network
-    - user
     - root
     - luks
   loop_control: