feat(css): globales CSS bearbeiten (lit/divi-custom-css-get und -update)

WordPress „Zusätzliches CSS“ (in Divi „Eigenes CSS“) über die Design-Pipeline: Probelauf mit Zeilen-Diff,
design_rev, Bestätigung, Snapshot, Rollback und Wiederherstellen (Bereich custom_css). Der Guard lehnt </style,
<script, expression(, javascript: und neue @import ab und warnt bei fremden Hosts; nur Neues zählt, Bestehendes
blockiert keine Änderung. Erfordert edit_css und die Divi-Rolle Theme-Anpassung.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Integrations/Divi/Design/CustomCssStore.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\Divi\Design;
+
+use RuntimeException;
+
+defined('ABSPATH') || exit;
+
+/**
+ * The site-wide custom CSS of the active theme: WordPress' "Zusätzliches CSS", a custom_css post per theme. Divi 5's
+ * field "Eigenes CSS" writes the same post (themes/Divi/epanel/core_functions.php), so this is the one place. Not
+ * final so the area and the abilities can be tested with a mock.
+ */
+class CustomCssStore
+{
+    /**
+     * @return array{stylesheet: string, css: string} The stored text – not wp_get_custom_css(), which plugins filter.
+     */
+    public function read(): array
+    {
+        $stylesheet = get_stylesheet();
+        $post = wp_get_custom_css_post($stylesheet);
+
+        return ['stylesheet' => $stylesheet, 'css' => $post === null ? '' : (string) $post->post_content];
+    }
+
+    /** @throws RuntimeException When WordPress refuses to store the CSS. */
+    public function write(string $css): void
+    {
+        $saved = wp_update_custom_css_post($css, ['stylesheet' => get_stylesheet()]);
+        if (is_wp_error($saved)) {
+            throw new RuntimeException($saved->get_error_message());
+        }
+    }
+}