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

tests/Unit/Integrations/WordPress/WordPressIntegrationTest.php

@@ -18,6 +18,8 @@ final class WordPressIntegrationTest extends TestCase
         'lit/content-types',
         'lit/content-query',
         'lit/content-get',
+        'lit/content-create',
+        'lit/content-update',
     ];
 
     public function testProvidesTheWordPressAbilitiesInOrder(): void