docs: update to v1.2 with SMS delivery feature documentation
- design.md v1.2: add section 2.5 (SMS delivery via dfsle.Recipient.withSmsDelivery());
document SMS_PLACEHOLDER_EMAIL constant, resolveRecipients/buildRecipient signatures,
Flow V4 structure and decision paths; update component diagram and Request class docs
- requirements.md v1.2: add FR-007 (SMS delivery for recipients without email),
US-006 user story; update constraints and success criteria
- api-reference.md v1.2: add section 11 (SMS delivery via dfsle toolkit);
document recipientSmsPhone parameter, withSmsDelivery() usage pattern,
Flow V4 integration steps; renumber Reference Links to section 12
71a156df78