Constant EVAL_SIBLING_PREFIX 

pub const EVAL_SIBLING_PREFIX: &str = "eval_";

The eval_ prefix of the sibling-suite naming convention: a lesson lessons/foo.yaml pairs with the suite lessons/eval_foo.yaml. The single source of the convention — the scaffolding derives the sibling name through eval_sibling_path, and cli’s sibling_suite_path (which eval and eval-report resolve suites by) delegates to the same function, so the two ends of the convention can never drift apart.