feat: add system information context to prompt

[Amber-Williams]

I noticed the command by the LLM wasn't matching what I needed for my system so I've been modifying my prompt using an alias function like this with better results. With this feature llm-cmd will improve the plugin for others too.

ask_cmd() {
    SYSTEM_PROMPT="Return only the command to be executed as a raw string, no string delimiters wrapping it, no yapping, no markdown, no fenced code blocks, what you return will be passed to subprocess.check_output() directly.

    The user's system output from uname -a is:
    $MY_SYSTEM_CONTEXT

    For example, if the user asks: undo last git commit
    You return only: git reset --soft HEAD~1"

    llm cmd --system "$SYSTEM_PROMPT" "$*"
}

Warning

I wasn't able to run the tests

[ohEmily]

Could #18 be why you couldn't run the tests? pytest worked for me from a virtualenv once I added that line.