cmux: open a remote cmux-tui terminal as a native tab, not nested

Running cmux-tui attach --session S --terminal ID from a shell that is already a cmux-tui-backed pane draws the target terminal inside a bordered inner grid (it looks like cmux inside cmux), and a plain attach --session S gives you the whole nested TUI. The native route goes through the Mac app CLI: cmux ssh-session-list --all-workspaces lists every remote PTY (term_...) per workspace, then cmux ssh-session-attach --workspace <ws> --session-id term_... [--split right] --focus false opens it as a real Ghostty tab. Before using a terminal ID, check which host owns it with cmux-tui terminal list / client list on each candidate host, because session names like cmux-svedit can exist on several hosts. A terminal attached by several clients is sized to the smallest one, so detach any leftover inner attaches to get full width back.