Mention system for collaboration. Typing @ opens a project-member picker; saved content stores mention tokens, not just plain text. Mentioned users get notifications through the notification spine with respect for preferences and quiet hours.

Surfaces: public/admin comments, internal team comments, support ticket notes, docs suggestions, and changelog drafts. Guardrails: only members visible in private/admin contexts; public commenters cannot enumerate private member emails. Includes mention rendering, notification event type post.mentioned_you, and search indexing.