Here's my take on an LLM based shell helper for command suggestions and more: https://github.com/scottyeager/Pal

It features an integration with the abbreviation feature in fish and zsh (via plugin) to expand the suggestions for editing, if needed, and execution.

There's stdin support, which can be used to pipe in error messages, for example. I've thought about something like your tmux setup to capture and pass some history to the model, but in practice redirecting stderr handles most of the potential use case.