Function upsert_agent 

pub fn upsert_agent(
    storage_root: &Path,
    agent: AgentDefinition,
) -> Result<(), String>

Insert or update agent in the stored roster, keyed by slug, then persist.

If an agent with the same slug exists it is replaced in place (preserving its original created_at_unix); otherwise agent is appended. updated_at_unix is bumped to the current wall-clock in both cases. Because load_roster seeds the default local agent when the store is empty, the first upsert of a new agent also materialises that seed to disk.