refactor: DiviWriteAbility base and shared DiviErrors (H4)

The five Divi write tools extend DiviWriteAbility (final permission and
execute, change() per tool, shared output schema). DiviErrors holds the
unknown-node, position and compose-error mapping; DiviContentComposer uses
the same mapping (ledger T9). ToolNamingTest skips abstract classes.

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

src/Integrations/Divi/Pages/DiviPageLoader.php

@@ -12,7 +12,6 @@ use LIT\MCP\Integrations\WordPress\PostGuard;
 use LIT\MCP\Integrations\WordPress\Schema\ContentSchema;
 use LIT\MCP\Log;
 use WP_Error;
-use WP_Post;
 
 /**
  * Loads an entry for the Divi tools: rights and status via PostGuard (spec §4.5), then the
@@ -91,12 +90,4 @@ class DiviPageLoader
 
         return PostGuard::statusStable($post);
     }
-
-    public static function unknownNode(string $id, WP_Post $post): WP_Error
-    {
-        return Errors::notFound(
-            sprintf('Knoten "%s" gibt es in Eintrag %d nicht.', $id, $post->ID),
-            self::OUTLINE_HINT
-        );
-    }
 }