feat: Facebook Messenger manage-chat DM path

[TomGranot]

Instagram comment-to-DM already exists (private reply on comment_id). There is no Facebook Messenger / pages_messaging / manage_chat path.

Need:

• Document the Meta app permission set for Instagram private replies AND Facebook Page manage-chat / DMs
• Extend the reference worker so a matching comment (or equivalent Facebook CTA) can DM the source URL on both Instagram and Facebook
• Keep the existing live-write locks (MODE, WRITE_MODE, confirmation phrase, unlock file). No automatic retry of uncertain sends.

This is a human-gated Meta app change plus the worker. Do not scrape cookies or unofficial inboxes.