Function quickstart 

pub fn quickstart(
    directory: &str,
    key: &str,
    artifact_type: &str,
) -> Result<QuickstartResult, ScaffoldError>

quickstart(directory, key, artifact_type) — validate the type first, refuse a non-empty corpus BEFORE any write, establish identity, then scaffold <dir>/decisions/<type>s/first-<type>.md.

Check order is load-bearing (measured): bad type (exit 2) beats a non-empty corpus (exit 1) beats a bad key (exit 2 when reached). Note the identity write lands BEFORE the starter-exists refusal, exactly like the oracle (init_repository precedes create_artifact).