Function open_pane 

pub async fn open_pane(
    state: &AppState,
    cmd: CommandPaneOpenData,
) -> Result<PaneOpenResult, String>

Core pane.open logic, shared by the WebSocket RPC handler (register_pane_open) and the HTTP route POST /api/v1/pane/open (agentmux-mcp’s OpenEditor tool). Creates a block for the requested view, enqueues a layout action (split or insert), and broadcasts the block/tab/layout updates so the frontend renders the new pane.