Include operating system in the AI system prompt to improve command quality

[bcantoni]

Fixes #19

[bcantoni]

Still testing but this seems to work pretty well (testing on Mac with gpt-4o-mini).

Before

$ llm cmd check for all ports listening on tcp
> netstat -tuln | grep LISTEN

After

$ llm cmd check for all ports listening on tcp
> lsof -nP -iTCP -sTCP:LISTEN

[mpetruc]

Do you know what's the status on this PR? Any hint of when it might get merged? Thank you so much for implementing this!