/**
 * Accept a credential value only when it is a literal secret that can be sent
 * verbatim in a header. Control bytes would make the request throw rather than
 * surface a credential problem, and `$`/`!` prefixed values are environment,
 * template, or command references that must never be resolved or executed.
 *
 * @param value candidate credential read from a local auth file
 * @returns the literal secret, or undefined when it is unusable
 */
export declare function usableLiteralSecret(value: unknown): string | undefined;