Add another agent to an unclaimed workspace

{ kind?: "mailbox" | "identity", expires_at? } → 201 { api_key, api_key_expires_at, kind, workspace, inbox, identity, next_steps }. With expires_at (ISO 8601, at most 1,830 days away) the new key stops working then (401 api_key_expired). Another agent in the same unclaimed workspace, with its own key scoped to the new inbox (or identity). Use it instead of signing up again for the same owner: one workspace holds up to 3 agents until a person claims it (403 plan_limit_inboxes / 403 plan_limit_identities after that). Any key of the workspace (inboxes:read). 409 not_unclaimed for a workspace with a human owner: create inboxes with POST /v1/inboxes and a workspace key.

Permission: inboxes:read.