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/PlatformSystemRoles.php

@@ -11,10 +11,12 @@ final class PlatformSystemRoles
 
     public const SLUG_AGENT = 'agent';
 
+    public const SLUG_SITE_ADMIN = SitePlatformRole::SLUG;
+
     /** @return list<string> */
     public static function fixedSlugs(): array
     {
-        return [self::SLUG_SUPER_ADMIN, self::SLUG_AGENT];
+        return [self::SLUG_SUPER_ADMIN, self::SLUG_SITE_ADMIN, self::SLUG_AGENT];
     }
 
     public static function isFixedSlug(string $slug): bool
@@ -49,6 +51,7 @@ final class PlatformSystemRoles
     public static function ensureAll(): void
     {
         self::ensureSuperAdminRole();
+        SiteAdminDefaultRolePermissions::ensurePlatformSiteAdminRole();
         AgentDefaultRolePermissions::ensurePlatformAgentRole();
     }
 }