Function clear_blocker_read_cache 

pub async fn clear_blocker_read_cache(
    store: &TaskStore,
    repo: &str,
    external_id: &str,
)

Forget when a blocker was last read, so the next refresh reads it at once. Called when a task reaches done. Without it a blocker that finishes right after a read holds its dependents for the rest of the TTL, and a chain pays that at each step. A blocker orch does not manage has no task to finish, so it keeps the full TTL