Function stale_sign_in_reason 

pub(crate) fn stale_sign_in_reason(
    broker: Option<&CredentialState>,
    creds: &MuxBusCredentials,
    discovered: Option<&CloudSettings>,
) -> Option<String>

Why the stored AgentMux Cloud sign-in can’t work anymore, or None if it still can (docs/specs/SPEC_CLOUD_SETTINGS_DISCOVERY_2026_09_27.md §3.3). Stale means only a new sign-in helps:

• the cloud publishes a different desktop client id than the one the credential was issued to (it belongs to a user pool the cloud has left);
• or the broker’s refresh was refused outright (NeedsReauth with rejected), and the stored token isn’t fresh. A fresh one means someone signed in since, possibly from another channel sharing the store; the broker catches up on its next check.

Transient failures that merely piled up into NeedsReauth are not stale: the network coming back can still cure them.