Page-split now creates independent sessions (no parent_session_id), parent marked as status='split' and hidden from list. Navigation uses useSearchParams for URL-based step tracking (browser back/forward works). page.tsx reduced from 684 to 443 lines via usePipelineNavigation hook. Box sub-sessions (column detection) remain unchanged. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>