refactor: small Divi tree helpers out of DiviWriter (M3)

Block::isPlaceholder replaces the divi/placeholder literals (ledger T4);
NodeIndex::parentBlock/sectionLabel replace the two parentBlock copies and
DiviWriter::sectionLabel; DiviChange::idOf and Outline::summary replace the
duplicated id resolution and outline text; DiviSchema holds the shared
input properties. Behaviour unchanged.

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

src/Integrations/Divi/Pages/DiviDuplicateNodeAbility.php

@@ -34,7 +34,7 @@ final class DiviDuplicateNodeAbility extends DiviWriteAbility
     {
         return [
             'type' => 'object',
-            'properties' => array_merge(DiviWriter::inputProperties(), [
+            'properties' => array_merge(DiviSchema::write(), [
                 'id' => ['type' => 'string', 'description' => 'Pfad-ID des zu duplizierenden Knotens.'],
             ]),
             'required' => ['post_id', 'tree_rev', 'id'],