Cherry-pick: extensions-move WhatsApp (155 files)

## Extensions-move: WhatsApp adapter

**Upstream commit**: `16505718e8`
**Files**: ~155
**Decomposed from**: #2018

### Conflict Resolution
Move `src/whatsapp/` → `extensions/whatsapp/src/`. Keep fork's rebranded content.

### Exit Criteria
- `pnpm typecheck` passes
- No `openclaw` references in moved files