Removed the cmux, zellij, and wezterm backends, the MuxBackend type, the PI_SUBAGENT_MUX override, and dead exports (renameCurrentTab, renameWorkspace, sendEscape, isFishShell, exitStatusVar). The ten-function surface API used by index.ts is unchanged. Integration harness and tests are tmux-only; README and package.json now state tmux-only with a fork note pointing multi-multiplexer users at the upstream project.