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/DiviOutlineAbility.php

@@ -143,7 +143,7 @@ final class DiviOutlineAbility implements Ability
             return Errors::invalidInput('id ist keine Pfad-ID.', ['id' => 'z. B. "s0.r1.c0.m2"']);
         }
         if ($rootId !== null && $page->index->find($rootId) === null) {
-            return DiviPageLoader::unknownNode($rootId, $page->post);
+            return DiviErrors::unknownNode($rootId, $page->post);
         }
 
         $outline = new Outline($this->design->read()->presetNames());