Add or update a single operator secret field.
Merges into the operator's stored credentials without touching the
others — the field-level counterpart to re-delivering the whole
bundle over Secure Courier. Use it to rotate one secret (a reissued
``btcpay_api_key``, say) without restating the six you did not
change, where any field omitted from a courier reply is destroyed.
The value is never echoed back. RESTRICTED to the operator —
requires proof (nsec-signed kind-27235 or a cached dpop_token
phrase); patron proofs are rejected.
ConnectorNo auth