add a CLI for analyzing previous command return values

[covercash2]

maybe there's a way to analyze the return value of the previous command? it would require access to the inner workings of the shell, but it could be super useful to be able to do something like:

$ sudo nixos-rebuild ...
oops some bad things happend :(
$ djinn explain nix
Here's what you did idiot:
...

where djinn is the main CLI entrypoint, explain is the command to run, and nix is the agent that provides context for the answer.