feat: content-create and content-update with schema validation

All-or-nothing writes: validation (fields, terms, featured image, parent,
template) before any write; failed extras delete the new draft (create) or
restore revision, old columns, fields, terms and featured image (update).
content-update requires rev; Rev::ofPost covers content, title, excerpt,
status and post_modified_gmt. MetaFieldSource::write() now throws on failure.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/WordPress/Content/PostPresenter.php

@@ -61,7 +61,7 @@ class PostPresenter
             'terms' => JsonMap::of($this->terms($post)),
             'fields' => JsonMap::of($this->fields($post)),
             'featured_image' => $this->featuredImage($post),
-            'rev' => Rev::of((string) $post->post_content),
+            'rev' => Rev::ofPost($post),
             'hints' => $divi ? [self::DIVI_HINT] : [],
         ]);
     }