The native arboard backend cannot retain X11 / Wayland selection ownership
after the calling process exits, and for short-lived napi calls it can drop
ownership before any consumer sees the selection — leaving the clipboard
empty even though set_text returned success. tmux + QTerminal made this
visible because OSC 52 is also dropped by libqtermwidget, so both backends
fail and the UI's 'Copied to clipboard' status reads as a lie.
utils/clipboard.ts now:
- Tries wl-copy / xclip / xsel before arboard on Linux. These CLIs fork
after reading stdin and serve the selection until another app claims it,
so the payload survives our process exit.
- Honors OMP_CLIPBOARD_COMMAND as a shell-string escape hatch (e.g.
`xclip -selection clipboard -in -silent`).
- Logs a single warning when every backend fails, so the silent-success
regression from #2075 cannot recur unnoticed.
Tests assert dispatch order: Linux+X11 prefers xclip, Wayland prefers
wl-copy, xclip→xsel fallback works, all-fail falls back to native,
macOS still goes straight to native, and OMP_CLIPBOARD_COMMAND wins
over the CLI chain.
Fixes#2075
