Append a message to a session

Append a message to a session. On the first message the session is lazily created and the initiator is recorded as a participant. A human agent who is not yet a participant may append to an unclaimed (owner-less) cortex warm-start session on a ticket they can access — this claims it (they become its first participant), the discover-and-continue path. Posting to a session already claimed by another human returns 403. Auth rules: - Cookie/JWT (human agent): sender = , direction = ‘in’ - Internal token (cortex/NEO): sender = null, direction = ‘out’; costUsd, usageJson, model accepted only from the internal caller. The to field is stamped by Mind from the current participant set — never read from the request body.