- Remove -- separator from tmux args (psmux on Windows doesn't support it)
- Unknown subcommands now show error + usage (exit 1) instead of starting TUI
- --help now always shows usage and exits (no fallthrough to TUI)
tmux aborts the entire command chain when kill-session fails (no
existing session). This caused 'no current target' error on Linux.
The kill-session is now a separate exec.Command() call before
starting the tmux chain, ignoring any error.