Function delete 

pub fn delete(
    identity_id: &str,
    origin: &str,
    realm: &str,
    is_proxy: bool,
) -> Result<(), String>

Delete the stored credential for this identity + protection-space. Idempotent (a missing entry is success) — same contract as secret_store::delete. Called when a stored credential turns out to be wrong (a re-challenge shortly after an auto-fill — see the credential_broker orchestration in agentmux-cef).