fix: check WordPress meta capabilities per field key

Fields from the meta/sample sources need edit_post_meta (update, checked in
PostWriter::prepare) or add_post_meta (create, checked right after the insert;
a refusal deletes the new draft again) for their key; a refusal is a
fields.<key> validation error.

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

src/Integrations/WordPress/Content/ContentCreateAbility.php

@@ -118,6 +118,11 @@ final class ContentCreateAbility implements Ability
         }
 
         $postId = (int) $postId;
+        $denied = $this->writer->deniedMeta($postId, $postType, $prepared['fields'], 'add_post_meta');
+        if ($denied !== []) {
+            return $this->refuse($postId, $postType, $denied);
+        }
+
         try {
             $this->writer->writeExtras($postId, $postType, $prepared, $input);
         } catch (Throwable $e) {
@@ -187,6 +192,37 @@ final class ContentCreateAbility implements Ability
         );
     }
 
+    /**
+     * Meta rights can only be checked once the draft exists (add_post_meta needs its ID): a
+     * refused field removes the brand-new draft again and is reported like any invalid input.
+     *
+     * @param array<string, string> $denied fields.<key> => error
+     * @throws WriteFailed When the draft cannot be deleted.
+     */
+    private function refuse(int $postId, string $postType, array $denied): WP_Error
+    {
+        try {
+            $deleted = (bool) wp_delete_post($postId, true);
+        } catch (Throwable $e) {
+            $deleted = false;
+        }
+
+        if (! $deleted) {
+            $summary = sprintf('Entwurf %d: Anlegen nicht zurückgenommen', $postId);
+            $this->logPartial($postType, (string) $postId, $summary);
+            throw new WriteFailed(
+                'Keine Berechtigung für ' . implode(', ', array_keys($denied)) . '.',
+                false,
+                $postId,
+                null,
+                null,
+                sprintf('Meta rights denied for new post %d; draft could not be deleted', $postId)
+            );
+        }
+
+        return Errors::invalidInput('Eingaben sind ungültig; nichts wurde gespeichert.', $denied);
+    }
+
     private function logPartial(string $postType, string $postId, string $summary): void
     {
         $this->log->recordPartial($this->name(), $postType, $postId, $summary);

src/Integrations/WordPress/Content/PostWriter.php

@@ -33,6 +33,11 @@ class PostWriter
         'template' => 'page_template',
     ];
 
+    /** Field sources that store plain post meta, guarded per key by WordPress' meta capabilities. */
+    private const META_SOURCES = ['meta', 'sample'];
+
+    private const META_DENIED = 'Keine Berechtigung, dieses Feld zu ändern.';
+
     /** wp_insert_post() ignores an empty page_template; this value selects the default template. */
     public const DEFAULT_TEMPLATE = 'default';
 
@@ -91,6 +96,12 @@ class PostWriter
             $errors['fields.' . $key] = $message;
         }
 
+        // On create the entry does not exist yet; lit/content-create checks add_post_meta after the insert.
+        $postId = (int) ($input['post_id'] ?? 0);
+        if (! $isCreate && $postId > 0) {
+            $errors = array_merge($errors, $this->deniedMeta($postId, $postType, $fields['values'], 'edit_post_meta'));
+        }
+
         $terms = $this->terms->resolve($postType, is_array($input['terms'] ?? null) ? $input['terms'] : []);
         foreach ($terms['errors'] as $taxonomy => $message) {
             $errors['terms.' . $taxonomy] = $message;
@@ -110,6 +121,33 @@ class PostWriter
         return ['fields' => $fields['values'], 'terms' => $terms['terms'], 'warnings' => $fields['warnings']];
     }
 
+    /**
+     * Meta fields (sources meta/sample) the current user may not write: WordPress decides per
+     * key (protected keys, register_meta auth callbacks) via edit_post_meta / add_post_meta.
+     *
+     * @param array<string, mixed> $values Validated field values.
+     * @param string               $cap    edit_post_meta (update) or add_post_meta (create).
+     * @return array<string, string> fields.<key> => error
+     */
+    public function deniedMeta(int $postId, string $postType, array $values, string $cap): array
+    {
+        $definitions = $this->schema->fields($postType);
+        $denied = [];
+        foreach (array_keys($values) as $key) {
+            $key = (string) $key;
+            $definition = $definitions[$key] ?? null;
+            if (
+                $definition !== null
+                && in_array($definition->source, self::META_SOURCES, true)
+                && ! current_user_can($cap, $postId, $key)
+            ) {
+                $denied['fields.' . $key] = self::META_DENIED;
+            }
+        }
+
+        return $denied;
+    }
+
     /**
      * @param array<string, mixed> $input
      * @return array<string, mixed>