Proposal: store OpenAI API key securely

Nobody has claimed this yet.

I think that exposing secrets such as API keys to persistant shell variables is not safe.

One solution is to add this alias-like function to `.bashrc/.zshrc`
```sh
# API key is in the global scope only during this command's execution
function pls() { # called "pls" so that it does not clash with "plz"
export OPENAI_API_KEY=$(<~/.openai_api_key) # API key is stored in a separate file
plz $1
unset OPENAI_API_KEY
}
```

The other solution is to implement safe storage and retrieval in "plz" itself. Maybe using an OS native solution like "keychain" in OSX.
But I feel like this is too much to ask for.

Sadly I am unfamiliar with rust. So, I can't contribute right away.

Start by reviewing the proposed .bashrc/.zshrc function and how plz handles OPENAI_API_KEY. Compare temporary shell exposure with native secure storage such as the macOS Keychain. Done requires an agreed storage and retrieval design before implementation can be scoped.

Written by the indexing model from the issue text.