feat: media usage finder and media-get tool

Usage search distinguishes certain (ID referenced) and possible (file URL
only) usages, hides titles the user cannot read, and keeps queries bounded.
MediaGuard checks per-attachment capabilities and refuses trashed media
unless the caller opts in.

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

src/Integrations/WordPress/Schema/ContentSchema.php

@@ -130,6 +130,29 @@ class ContentSchema
         throw new LogicException(sprintf('No field source "%s" registered.', $field->source));
     }
 
+    /**
+     * Keys of fields that store attachment IDs (file fields, relations to media) over all
+     * managed post types, sorted – used by the media usage search.
+     *
+     * @return list<string>
+     */
+    public function mediaFieldKeys(): array
+    {
+        $keys = [];
+        foreach ($this->postTypeNames() as $postType) {
+            foreach ($this->fields($postType) as $field) {
+                if ($field->type === 'file' || ($field->related['object_type'] ?? null) === 'media') {
+                    $keys[$field->key] = true;
+                }
+            }
+        }
+
+        $keys = array_map('strval', array_keys($keys));
+        sort($keys);
+
+        return $keys;
+    }
+
     /** @return list<array{name: string, label: string, hierarchical: bool}> */
     public function taxonomies(string $postType): array
     {