Set Brain

Persist this account's brain choice (v2 §4.7). Since owner ruling D8 (2026-10-06) a stored choice no longer changes the brain a run takes (the default, or the Deep cut for a run sent with engine: "deep"); the call is kept so existing clients do not break. A key whose principal carries no email is refused with feature_not_entitled. Costs nothing quota-wise. Requires the account:write scope (owner-gated, not minted by default): a key without it is refused with scope_missing, not a 404. Send config as one of the exact ids from GET /v1/brains; an unknown id is rejected with unknown_brain. Recover from unknown_brain by reading GET /v1/brains again; from feature_not_entitled by not calling this on an ineligible account; for service_unavailable, wait retry_after_sec and retry the same request once.