Store a memory

Stores a new memory entry, with the authenticated agent as its source. `scope` names the audience, and each scope requires the field that identifies it: - `agent` — private to this agent. The default when no `scope` and no `subject_id` is given, so an unqualified write is private rather than shared. - `subject` — readable by any agent in the organization dealing with `subject_id`, which is required. The default when a `subject_id` is given without a `scope`. - `organization` — readable by every agent in the organization. Rejected with 422 when the agent belongs to none, since there would be no audience to share it with.