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>

inventory_libvirt_example.yml

@@ -39,11 +39,11 @@ all:
                 mount:
                   path: /var/www
                   fstype: xfs
-            user:
-              name: "web"
-              password: "CHANGE_ME"
-              keys:
-                - "ssh-ed25519 AAAA..."
+            users:
+              - name: "web"
+                password: "CHANGE_ME"
+                keys:
+                  - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             packages:
@@ -81,11 +81,11 @@ all:
                 mount:
                   path: /data
                   fstype: ext4
-            user:
-              name: "db"
-              password: "CHANGE_ME"
-              keys:
-                - "ssh-ed25519 AAAA..."
+            users:
+              - name: "db"
+                password: "CHANGE_ME"
+                keys:
+                  - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             luks:
@@ -122,11 +122,11 @@ all:
                 mount:
                   path: /data
                   fstype: btrfs
-            user:
-              name: "compute"
-              password: "CHANGE_ME"
-              keys:
-                - "ssh-ed25519 AAAA..."
+            users:
+              - name: "compute"
+                password: "CHANGE_ME"
+                keys:
+                  - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             features: