Function grant_issue 

pub fn grant_issue(
    cfg: &HubConfig,
    brain: &str,
    grantee: &str,
    can: Capability,
    scope: Option<&str>,
    until: Option<&str>,
) -> LinkResult<Value>

Issue (or refresh) a grant on brain to grantee — a hub principal named by email in v0 (the protocol’s near-term simplification; key-named grantees arrive with the signing layer). scope is a store-path prefix (the hub’s enforcement unit); until an ISO 8601 expiry, absent = until revoked.