feat(tmux): deploy the config on Linux too; prefix i / - splits

- install.sh: deploy_tmux_config runs on both OSes after the bashrc (gated on tmux)
- tmux.conf: clipboard via if-shell (pbcopy on macOS, tmux buffer + OSC 52 elsewhere)
- bashrc-linux exports XDG_CACHE_HOME like the macOS rc files
- prefix i = side-by-side split, prefix - = stacked split (h/j/k/l kept)
- verified in an Ubuntu 24.04 container (tmux 3.4): deploy, bindings, split, plugins, libtmux

bash/bashrc-linux

@@ -7,6 +7,9 @@ alias grep='grep --color=auto'
 # colored GCC warnings and errors
 export GCC_COLORS='error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01'
 
+# XDG cache dir: tmux.conf saves resurrect/continuum sessions under it
+export XDG_CACHE_HOME="${XDG_CACHE_HOME:-$HOME/.cache}"
+
 # Ensure ~/.local/bin is on PATH (pipx CLIs + personal scripts from bin/)
 case ":$PATH:" in
 	*":$HOME/.local/bin:"*) ;;