Redeem an invitation code. NO authentication (1.62.0).

Body: code, agent_name, client_name?. Creates an agent identity in the inviter's workspace with exactly the invitation's capabilities and scopes and returns credential { api_key (fbzr_ renewal credential), token_type, header, scopes, capabilities, expires_at, renewable, token_exchange } once, plus handoff. Single use: 410 INVITATION_ALREADY_USED | INVITATION_EXPIRED | INVITATION_REVOKED with a plain reason; 404 INVITATION_NOT_FOUND; 429 REDEEM_RATE_LIMITED (per IP and per code).