Changelog
- Added LinkedIn page mention validation: rejects posts where the mention text doesn't match the organization's exact registered name on LinkedIn (case-sensitive), preventing silent plain-text fallback
- Added Facebook mention validation: rejects posts where the anchor text is missing from comment containers, or where the target page ID is inaccessible
- Moved Facebook page tagging from the post body to comment containers — Meta's Graph API only supports @[pageId] mentions in comments, not feed posts
- facebook.mentions now requires at least one comment container (second containers entry) whose text contains the anchor; posting mentions without a comment container returns a clear error