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/Abilities/Rev.php

@@ -4,13 +4,32 @@ declare(strict_types=1);
 
 namespace LIT\MCP\Abilities;
 
+use WP_Post;
+
 /**
- * Short hash of post_content – the optimistic-lock handle ("rev", spec §5.3).
+ * Short hash used as optimistic-lock handle ("rev", spec §5.3).
  */
 final class Rev
 {
+    /** Hash of a single string, e.g. post_content (Divi layouts). */
     public static function of(string $content): string
     {
         return substr(hash('sha256', $content), 0, 12);
     }
+
+    /**
+     * Rev of an entry for the content tools: changes when content, title, excerpt or
+     * status change, or when anything else touched the post row (post_modified_gmt),
+     * so parallel edits beyond post_content are detected as well.
+     */
+    public static function ofPost(WP_Post $post): string
+    {
+        return self::of(implode("\0", [
+            (string) $post->post_content,
+            (string) $post->post_title,
+            (string) $post->post_excerpt,
+            (string) $post->post_status,
+            (string) $post->post_modified_gmt,
+        ]));
+    }
 }