Get Agent Counsel Context

Snapshot-scoped context for an agent-service agent-counsel turn (the branch loader).

Agent-service-facing (caps=['agent-counsel-turn']). The reviewer (=user_id) and snapshot_id come from the verified JWT, never request input. Authorization is the cap plus an open ReviewRequest for (reviewer_id, snapshot_id); never workspace membership, since synthetic reviewers are not members. Returns the one snapshot's content, the workspace identity + connected sources, and the comments on that snapshot. turns is empty: agent counsel is a single fire-and-forget pass with no private thread to walk.