feat: lit/content-create with Divi short form

New input divi (1-30 sections, short form like lit-divi-insert) builds a Divi 5
layout in one divi/placeholder. The whole description is checked before the
insert (Divi's map for "no entry yet"); the draft is inserted with its normal
WordPress fields only, then DiviContentComposer composes again bound to the new
entry and stores it via DiviSaver::saveNew (same kses window as save(), strict
DiviFlags). A refusal or failure deletes the draft. Result adds tree_rev and the
outline.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/Integrations/WordPress/Content/ContentComposer.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\WordPress\Content;
+
+use Throwable;
+use WP_Error;
+
+/**
+ * Builds the content of a new entry for lit/content-create from a structured description
+ * (spec §6.2: "optional Divi-Inhalt in Kurzform"). Implemented by the Divi integration, so the
+ * WordPress integration does not depend on Divi.
+ *
+ * Two steps, because the entry does not exist before the insert: check() validates the whole
+ * description before anything is created (all or nothing); write() stores the content into the
+ * brand-new draft, which lit/content-create inserted with its normal WordPress fields only.
+ */
+interface ContentComposer
+{
+    /** Whether composing is possible on this site right now (e.g. Divi 5 active). */
+    public function available(): bool;
+
+    /**
+     * Validates the description for a new entry of $postType. Nothing is written.
+     *
+     * @param list<mixed> $sections
+     * @return WP_Error|null Every problem of the description; null = valid.
+     */
+    public function check(string $postType, array $sections): ?WP_Error;
+
+    /**
+     * Composes the description for the new draft $postId and stores it. A WP_Error means the
+     * description is invalid for this entry (nothing was written) – like a throw, it makes
+     * lit/content-create delete the draft again.
+     *
+     * @param list<mixed> $sections
+     * @return array{warnings: list<string>, outline: ?string}|WP_Error
+     * @throws Throwable When storing failed.
+     */
+    public function write(int $postId, string $postType, array $sections): array|WP_Error;
+}