CommandHandlerSpec()_ConfigModelConfiguration for a synchronous command hook.
Currently only type: "command" is supported. Additional handler types
remain a discriminated-union extension point and are rejected until
implemented.
When argv is set, the runner launches via create_subprocess_exec and
ignores shell metacharacters in command.
argv is a temporary legacy-migration compatibility field. Remove it with
hooks.legacy and hooks.migration after September 1, 2026.