feat(social): implement WhatsApp Business tools — send messages

[EngineerProjects]

Context

Stub is at `internal/tools/social/whatsapp/tool.go`. All methods return `ErrNotImplemented`. `IsEnabled() = false`.

Tools to implement

Tool	Description
`whatsapp_send`	Send text or approved template message
`whatsapp_send_media`	Send image/document/audio/video
`whatsapp_send_interactive`	Send button or list messages

Auth + prerequisites

WhatsApp Business Cloud API (Meta for Developers):

1. Create a Meta Business account
2. Create an app with WhatsApp product
3. Add a phone number (test numbers available in dev mode)
4. Get permanent token via System User in Business Settings

Required env vars:

• `WHATSAPP_PHONE_NUMBER_ID` — from Meta dashboard
• `WHATSAPP_TOKEN` — permanent access token
• `WHATSAPP_BUSINESS_ACCOUNT_ID` — for template management

API

Cloud API v20.0: `https://graph.facebook.com/v20.0/{phone-number-id}/messages\`

Key rules:

• Template messages: required for outbound-initiated conversations. Templates must be approved by Meta (24-72h review).
• Free-form messages: only allowed in response to a user message, within 24h window.
• Webhook: to receive inbound messages, a public HTTPS endpoint is needed.

Implementation notes

• `whatsapp_send` with `type=text`: only valid if user messaged first (check 24h window or add guard)
• `whatsapp_send` with `type=template`: always valid, requires `template_name` + `template_lang`
• All send tools require `RequiresPermission: true`
• Add `WHATSAPP_TEST_TO` env var for sandbox/test phone number
• Meta's test mode allows sending to up to 5 verified numbers without template approval