fix: add api_key_env support for OpenAI provider

[joekryan]

openai was the odd one out - gemini-api already supports api_key_env but openai only looked for api_key in config.

added resolveOpenAIKey function following the same pattern as resolveGeminiAPIKey. now you can use:

ai:
  providers:
    openai:
      model: gpt-5
      api_key_env: OPENAI_API_KEY

instead of hardcoding the key in the config file.

nice project btw, I've been using it all morning :)

[bgdnvk]

thanks, will have a look 2mrw and into the other PR