Rather than lowering the validation bar, improve the system prompt:
- Rule 6 now explicitly states next_node_id must match a direct child's id
- Added rule 10: build tree bottom-up to avoid forward-reference errors
- Corrective prompt now calls out the ID mismatch constraint specifically
Reverts the strict=False fallback — flows must be correct before saving.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>