- buildSystemPrompt() accepts disable_brand_rules to skip the Dutch global
  rules + merkverhaal + schrijfstijl, for technical English prompts.
- buildSystemPrompt() accepts brand_story and writing_style options that
  override the Customsetting fallbacks, so consumers can pass brand voice
  dynamically per call.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>