Function trigger_payload 

pub async fn trigger_payload(
    manifest: Option<&Path>,
    state_dir: &Path,
    name: &str,
    provider: &str,
    kind: &str,
    metadata_pairs: &[String],
    at: Option<&str>,
    cost_usd: f64,
    tokens: u64,
) -> Result<PersonaRunReceipt, String>

In-process variant of harn persona trigger. metadata_pairs accepts the same KEY=VALUE strings the CLI does.