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/DiviNodeGetAbility.php

@@ -118,7 +118,7 @@ final class DiviNodeGetAbility implements Ability
 
         $block = $page->index->find($id);
         if ($block === null) {
-            return DiviPageLoader::unknownNode($id, $page->post);
+            return DiviErrors::unknownNode($id, $page->post);
         }
 
         $names = $this->design->read()->presetNames();