Adds an opt-in terminal setting that automatically copies the current
selection to the system clipboard as the user selects, mirroring X11 /
gnome-terminal behavior. xterm.js has no native option for this, so the
renderer hooks `onSelectionChange` per pane and writes via the existing
clipboard IPC. Defaults to false so existing users keep the explicit
Cmd/Ctrl+Shift+C copy flow.
Closes#860