AiResponse extends with dueDate: string|null. zod regex
^\d{4}-\d{2}-\d{2}$, follow-up roundtrip check coerces invalid
dates (2026-13-99 etc.) to null. PROMPT_VERSION → 2: prompt now
takes todayKst arg, asks model to extract due_date as ISO or null.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>