Function layout_open_impl 

pub(super) async fn layout_open_impl(
    state: &AppState,
    req: CommandLayoutOpenData,
    trust: &TrustInputs,
) -> Result<LayoutOpenResult, String>

Add the file’s tabs to window_id’s workspace — or, with new_window, to a new workspace for the caller to open a window onto — then launch each agent into its placeholder through agent.open (spec §3.5). Nothing already open is touched. An agent that can’t start (not installed, live elsewhere, …) leaves its pane as the agent picker and adds a note.