Poll a v2 conversation for its status and the message tail since a seq

The HITL-04 §6.6 poll primitive — net-new, message+seq shaped.

Distinct from GET /{id}/turns (cursor-paginated over turns): this is the one round-trip a caller loops on while a turn is parked, returning the conversation's current session_status AND every message with seq > since_seq. The SAME primitive serves approval-resume polling (awaiting_approval → active) and handoff operator-relay polling (waiting_for_human → closed); the uniform waker generates+persists the resumed/operator message and the caller's next poll picks it up.

The caller watermarks on the returned next_seq (the max seq of the tail, or the request since_seq when the tail is empty so an idle poll never rewinds). Borrows the read-router message-row + worst-of-N delivery- status plumbing; ordered by seq ascending. Org-scoped: cross-org / missing id → 404 before any read.