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/DiviMoveAbility.php

@@ -35,9 +35,9 @@ final class DiviMoveAbility extends DiviWriteAbility
         return [
             'type' => 'object',
             'properties' => array_merge(
-                DiviWriter::inputProperties(),
+                DiviSchema::write(),
                 ['id' => ['type' => 'string', 'description' => 'Pfad-ID des zu verschiebenden Knotens.']],
-                DiviWriter::positionProperties()
+                DiviSchema::position()
             ),
             'required' => ['post_id', 'tree_rev', 'id'],
             'additionalProperties' => false,