Feature request: pluggable/custom backend for split-pane teammate mode

teammateMode currently supports split panes only on tmux and iTerm2, with the backend selection hardcoded. Terminal multiplexers that are not tmux (herdr, Zellij, WezTerm's mux, Ghostty once it grows an API) can only host teammates by shipping a fake tmux shim on PATH that translates the verbs — cmux (manaflow-ai/cmux) does this in production, and herdmates (caioniehues/herdmates) does the same for herdr. Both shims break on point releases because the tmux verb surface is an undocumented internal contract (e.g. compat fixes needed at 2.1.183, behavior changes around 2.1.212/2.1.234; the herdmates shim was most recently re-verified working on 2.1.237).

Ask: a pluggable backend — e.g. teammateMode: "command" plus a configured executable implementing a small documented verb set (create pane with cwd/title, send text, kill pane), or simply documenting the tmux verbs Claude Code emits as a stable contract. Precedent: the iTerm2 backend was added in a point release, so the seam already exists internally.