feat: index.php bootstrap migration from MokoCassiopeia

Joomla 6 doesn't call <scriptfile> for templates. Instead, run
migration on first page load via index.php:
- Detect MokoCassiopeia styles → copy params to MokoOnyx
- Create matching style copies for additional styles
- Set MokoOnyx as default if Cassiopeia was default
- Copy user files (custom themes, user.css, user.js)
- Redirect update server to MokoOnyx
- Creates .migrated marker so it only runs once

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/index.php

@@ -9,6 +9,11 @@
 
 defined('_JEXEC') or die;
 
+// One-time migration from MokoCassiopeia (runs once, creates .migrated marker)
+if (!file_exists(__DIR__ . '/.migrated')) {
+    require_once __DIR__ . '/helper/migrate.php';
+}
+
 use Joomla\CMS\Factory;
 use Joomla\CMS\HTML\HTMLHelper;
 use Joomla\CMS\Language\Text;