ISSUE-021 — WhatsApp port of winning nudge arm

[shivaram19]

• Functional surgeon view: After the Telegram experiment identifies a winning arm, the same nudge is sent via WhatsApp.
• Acceptance criteria:
  1. WhatsApp sender abstraction shares the same nudge payload format.
  2. CPAU is tracked and reported.
  3. Automatic fallback if one channel fails.
• Dependencies: ISSUE-018 (experiment conclusion).