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.