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>

vars_baremetal_example.yml

@@ -25,11 +25,11 @@ system:
       mount:
         path: /data
         fstype: ext4
-  user:
-    name: "admin"
-    password: "CHANGE_ME"
-    keys:
-      - "ssh-ed25519 AAAA..."
+  users:
+    - name: "admin"
+      password: "CHANGE_ME"
+      keys:
+        - "ssh-ed25519 AAAA..."
   root:
     password: "CHANGE_ME"
   luks: