The second interface (plan §9.1): an ACP v1 server so an editor can drive a Terret agent over JSON-RPC 2.0 on stdio. Newline-delimited framing, session/new spawns a durable agent, session/prompt pends the whole turn, session/update notifications projected from the session log. Consumes the same two seams the socket does with no change to core, which is the standing proof that the interface is not privileged. Zero runtime dependencies beyond stdlib json.