Feature Request: Add support for cmux to Studio

### What

I'd like to request we add [cmux](https://cmux.com) as a supported terminal application in the "Open in..." terminal selector. cmux is a free, open-source native macOS terminal built on Ghostty that's designed for AI coding agents, with vertical tabs, notification rings, split panes, and a scriptable socket API for automation.

### Why

cmux is gaining traction among developers who want split panes, session management, and a lightweight alternative to iTerm/Warp. It works really well with tmux and also with git worktrees so I've been leveraging it with Claude Code during my regular workday. Adding cmux would be a low-effort way to support more developer workflows.

### How

Based on the existing pattern in apps/studio/src/lib/is-installed.ts, this would involve:

1. Adding a cmux entry to `installationPaths.darwin` pointing to cmux.app
2. Adding the corresponding CLI launch command in the terminal handler

This follows the same pattern used for Ghostty, Warp, and iTerm.

Start in apps/studio/src/lib/is-installed.ts and inspect the existing Ghostty, Warp, and iTerm entries, then find the terminal handler used by the Open in... selector. Add cmux's darwin installation path and matching CLI launch command; done means cmux is detected and can be opened from the selector on macOS.

Written by the indexing model from the issue text.