claude-zai as an interactive tmux teammate

This is a lightweight replacement for the built-in teammate feature (TeamCreate / SendMessage), which only supports internal subagents. claude-zai is an external CLI — another Claude Code instance — so we coordinate via tmux instead.

Architecture

• My pane: where the user is talking to me (read from $TMUX_PANE, e.g. %29). Full pane target is $TMUX_PANE itself — tmux accepts bare pane IDs as targets.
• Teammate pane: a new tmux window in the same session running claude-zai interactively.
• Me → teammate: tmux send-keys -t <teammate-pane> -l "<msg>" && tmux send-keys -t <teammate-pane> Enter
• Teammate → me: tmux send-keys -t <my-pane> -l "<msg>" && tmux send-keys -t <my-pane> Enter — lands as a normal user-prompt turn for me.
• User → both: tmux attach to the session; Ctrl-b <n> to switch windows; type in either.

Spawning the teammate

1. Verify I'm in tmux: require $TMUX and $TMUX_PANE to be set. If not, fall back to claude-zai -p background mode and tell the user.
2. Pick a window name based on the task (e.g. zai-video, zai-auth). Keep it short, prefix with zai-.
3. Create the window in the current session (use -c <cwd> so it starts in the project dir):