/**
 * Write `data` to `file` with owner-only (0600) permissions.
 *
 * - `{ mode: 0o600 }` sets the permission when the file is newly created.
 * - `fs.chmodSync(file, 0o600)` also tightens any pre-existing file (the mode
 *   option is ignored by the OS when the file already exists).
 *
 * The chmod is best-effort — on platforms/filesystems that don't support POSIX
 * modes (e.g. some Windows setups) it may throw, which must never break a write.
 */
export declare function writeFileSecure(file: string, data: string | Uint8Array): void;