Agent-layer REST surface: spaces, capability grants, sessions, and handoffs over HTTP. Space and handoff creation are management operations (global bearer only); everything scoped to one space accepts a capability token checked per method through barrel_server_caps (GETs need read, mutations write, grant administration admin).

Spaces open with default runtime options: an encrypted space needs its key spec on open, which HTTP callers cannot supply in v1, so encrypted spaces are Erlang-API-only for now.