feat(macos): deploy tmux.conf + tpm plugins; cloudpex becomes an install-time offer

- tmux.conf (vi keys, C-a prefix, resurrect/continuum, window-name) → ~/.config/tmux/tmux.conf
  on macOS; tpm cloned, plugins fetched headlessly, libtmux installed non-fatally
- bashrc-osx / zshrc-osx export XDG_CACHE_HOME (session save dir of the config)
- Linux: install_cloudpex → offer_cloudpex ([y/N] with the /tmp and SSH offers)
- README + CLAUDE.md updated

bash/bashrc-osx

@@ -17,6 +17,9 @@ 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:"*) ;;