config file permissions should be more restrictive

[theimpostor]

Running the command gptcommit config set openai.api_key sk-... the first time produced a config.toml which had group and world read permissions enabled (0644). Since this file contains an API key, seems like a good idea to create it with user read permissions only (0600).

[zurawiki]

Great point! Happy to accept a pull request. I don't think it would be more than a couple of lines