I noticed that you're using macOS's Accessibility API to provide your autocompletions. That's a clever use, and it sidesteps a huge source of pain when trying to make terminals smarter (i.e., dealing with the TTY/PTY layers, terminal capabilities, &c).

My question, since you mentioned that Linux support is "not an afterthought": how do you intend to support Linux? Your architecture on macOS is essentially a (benign) keylogger that I'm liable to trust because of Apple's strong isolation of concerns/entitlements; no such isolation is (consistently) available across Linux distributions. I suspect that most developers on Linux won't want your proprietary client running as a logger over X11 or Wayland.