A hidden tmux client: tmux attach running on a PTY pair the app owns.
From tmux’s perspective this is a normal terminal, so attach triggers a
full repaint, PTY resize drives window size (window-size latest), and
keyboard bytes written to the master are real terminal input.
Data-driven agent-harness registry — harness definitions are DATA, not
code (spec: field-notes-design.md §V “Backend (registry, not enum)”).
Four known harnesses ship as compiled-in default specs; [harnesses.*]
config entries override (whole-spec replace) or extend the registry, so
adding a future harness requires zero Rust changes.