Support cmux as a teammateMode backend for agent teams

Summary

Add cmux as a supported teammateMode option for agent teams, alongside the existing tmux and iTerm2 backends.

What is cmux?

cmux is a native macOS terminal app built on Ghostty, designed for running multiple AI coding agents simultaneously. It provides:

• Split panes (horizontal and vertical)
• A socket API and CLI for programmatic control (create workspaces, send keystrokes, manage surfaces)
• Environment variables (CMUX_WORKSPACE_ID, CMUX_SURFACE_ID, CMUX_SOCKET_PATH) automatically set in each pane
• Notification rings when agents need attention
• GPU-accelerated rendering via libghostty

Why

cmux's socket API exposes the same primitives agent teams already use with tmux (split-window, send-keys, session management), making it a natural fit. Its notification system and agent-focused design make it particularly well-suited for agent teams.

Proposed behavior

• teammateMode: "cmux" — spawn teammates into cmux split panes via the socket API
• Auto-detect cmux when CMUX_SOCKET_PATH is set and teammateMode is "auto"