Function open_session 

pub async fn open_session(
    agent: &Arc<dyn AgentCore>,
    cwd: &Path,
    resume: Option<SessionId>,
) -> Result<Arc<dyn Session>>

Opens a session running directly on the local machine (both fs and shell use local backends, frontend is Cli). resume = Some(id) resumes that session; otherwise creates a new one.

Errors

Returns an error if load_session / create_session fails (session does not exist, duplicate id, cwd unavailable, etc.).