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_example.yml

@@ -42,11 +42,11 @@ all:
                   fstype: xfs
                   label: DATA
                   opts: defaults
-            user:
-              name: "ops"
-              password: "CHANGE_ME"
-              keys:
-                - "ssh-ed25519 AAAA..."
+            users:
+              - name: "ops"
+                password: "CHANGE_ME"
+                keys:
+                  - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             packages:
@@ -99,11 +99,11 @@ all:
                 mount:
                   path: /srv/data
                   fstype: ext4
-            user:
-              name: "dbadmin"
-              password: "CHANGE_ME"
-              keys:
-                - "ssh-ed25519 AAAA..."
+            users:
+              - name: "dbadmin"
+                password: "CHANGE_ME"
+                keys:
+                  - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             luks: