Function parse_trigger 

pub fn parse_trigger(
    event: &str,
    payload: &Value,
    cfg: &HooksConfig,
) -> Option<Trigger>

Parse a webhook payload into a Trigger, or None when the event does not match a trigger rule (the caller 202-ignores it with a decision log). event is the X-GitHub-Event header value; anything outside ACCEPTED_EVENTS is None here too, keeping the allowlist meaningful at the core as well as at the route.