feat: refactor super admin to use is_super_admin flag and enhance site deletion logic

- Changed super admin detection from role-based to `is_super_admin` flag in AdminUser model
- Added `requireDefaultAdminSiteId()` method to throw validation error when no integration site exists
- Enhanced site deletion to migrate platform role bindings to fallback site and auto-delete site-specific admin accounts
- Made agent line code optional with auto-generation fallback using `{site_code}-agent-{counter}` format

app/Support/AdminAgentNodeAccess.php

@@ -23,6 +23,22 @@ final class AdminAgentNodeAccess
                 ?? AdminSite::query()->orderBy('id')->value('id'));
         }
 
+        // Check if admin is a platform account (bound via admin_user_site_roles)
+        $accessibleSiteIds = $admin->accessibleAdminSiteIds();
+        if ($accessibleSiteIds !== null) {
+            // Platform account (site admin)
+            if ($requestedSiteId !== null && $requestedSiteId > 0) {
+                if (in_array($requestedSiteId, $accessibleSiteIds, true)) {
+                    return $requestedSiteId;
+                }
+                return null;
+            }
+
+            // Return first accessible site if no specific site requested
+            return $accessibleSiteIds[0] ?? null;
+        }
+
+        // Agent account (bound via agent node)
         $actor = AdminAgentScope::primaryAgentNode($admin);
         if ($actor === null) {
             return null;