Function largest_session_id 

pub fn largest_session_id(
    projects_dirs: &[PathBuf],
    slug: &str,
) -> Option<String>

The provider session id (jsonl stem) of the agent’s largest session across the candidate project roots (the account-wide default and, for identity-bound agents, the identity bundle). We pick the largest, not the newest, on purpose: an accidental fresh session (one started when a blank cross-channel open failed to resume) must never win over the real conversation. None when there are no session files under any root.