cmux hits v0.64 with session restore, subagents

cmux v0.64 transforms the terminal into a reliable agent workbench by solving the "state loss" problem of long-running tasks through persistent session restore for agent progress, terminal scrollback, and browser history. The update introduces native subagent pane mapping for Codex Teams that mirrors hierarchical layouts of specialized tools like Claude Code while remaining terminal-native. Built on libghostty for GPU-accelerated performance, the app now features visual "notification rings" for ambient status updates and a scriptable WebKit browser that allows agents to interact with documentation directly, bypassing external headless tools.