Function set_model_cooldown 

pub async fn set_model_cooldown(
    agent_name: &str,
    model: &str,
    duration_secs: u64,
    reason: &str,
)

Set a model cooldown with a custom duration (in seconds) and reason.

Generic model-cooldown setter shared by silence detection and the rate-limit relative-usage-window path. The reason is persisted with the cooldown (KV + in-memory entry) so operators can tell the two apart in orch cooldown list.