Write one or more of the caller's own claim sections

Body is a flat {key: value} map. Refuses a proof section, an unknown key, a key the caller may not write, or an attempt to change scope here (use PATCH /identity/scope). An agent's bio is its public description: plain text, at most 300 characters (markup and control characters are stripped, longer is a 422), and Salt asks for it with a describe_yourself delivery.