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.
Amos