feat: content schema with registered and sampled meta fields

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

src/Integrations/WordPress/Schema/MetaFieldSource.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Integrations\WordPress\Schema;
+
+/**
+ * Shared read/write via the post meta API.
+ */
+abstract class MetaFieldSource implements FieldSource
+{
+    public function read(int $postId, FieldDefinition $field): mixed
+    {
+        return get_post_meta($postId, $field->key, ! $field->multiple);
+    }
+
+    public function write(int $postId, string $postType, array $values, array $definitions): void
+    {
+        foreach ($values as $key => $value) {
+            $key = (string) $key;
+            if ($value === null) {
+                delete_post_meta($postId, $key);
+                continue;
+            }
+
+            $field = $definitions[$key] ?? null;
+            if ($field !== null && $field->multiple && is_array($value) && array_is_list($value)) {
+                delete_post_meta($postId, $key);
+                foreach ($value as $item) {
+                    add_post_meta($postId, $key, wp_slash($item));
+                }
+                continue;
+            }
+
+            update_post_meta($postId, $key, wp_slash($value));
+        }
+    }
+}