Get Review Chat Context

Snapshot-scoped context for an agent-service review-chat turn.

Agent-service-facing (caps=['review-chat-turn']). The JWT's snapshot_id and user_id (the reviewer) scope the entire response; request inputs cannot widen it. Returns the one snapshot's content, workspace identity + connected sources, comments on that snapshot only, and this private thread's ancestry. Deliberately never touches the workspace /context walk.