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/defaults/main.yml

@@ -40,10 +40,7 @@ system_defaults:
   path: ""
   packages: []
   disks: []
-  user:
-    name: ""
-    password: ""
-    keys: []
+  users: []
   root:
     password: ""
   luks: