Function render_cli_error 

pub fn render_cli_error(
    code: &str,
    message: &str,
    json_mode: bool,
    details: Option<&Value>,
) -> (ErrorStream, String)

Decide the target stream and serialized line for a CLI error. Pure — the routing decision is unit-testable without capturing process streams. Under --json the {code, message, details} envelope goes to stdout so the documented memstead <sub> … --json | jq -r .code recipe works on the error path (a stdout-only pipe); the human markdown form goes to stderr. code is the stable UPPER_SNAKE_CASE token (matching the wire envelope agents see over MCP); details carries the structured recovery payload under the details key.