Enhancement: return code relevant to the OS llm is operating on

By default `llm cmd` generates code specific to Linux.  For example:
`llm cmd show list of system users`
returns
`cat /etc/passwd`
regards of OS, whereas 
`llm cmd show list of system users on windows`
returns
`net user`

It would be good if the OS was detected by `llm cmd` and the code returned was specific to the OS without the need to specify the OS in the command

