Function sync_cycle_once 

pub async fn sync_cycle_once(
    client: &Client,
    db_path: &Path,
    local_agent_id: &str,
    peer_url: &str,
    api_key: Option<&str>,
    batch_size: usize,
) -> Result<()>

Run a single sync cycle against one peer — pull then push.

Lifted verbatim (modulo path-of-Path-vs-PathBuf) from the pre-W6 main.rs::sync_cycle_once so the integration sync-daemon test can drive it without subprocess. The signature matches the private main.rs helper 1:1 to keep call sites identical.