Function add_history 

pub(crate) fn add_history(
    fs: &FileStore,
    agent_uid: &str,
    rows: &[ImportedVersion],
) -> Result<usize, StoreError>

Add rows to the agent’s history — no head changes, content the record already has for that file skipped. For versions kept beside a head the caller doesn’t replace (adoption of earlier accounts’ memory, §2.1.4). Returns how many were added.