[ai] Global on/off switch for LLM usage

[mmacy]

• Simplified enable/disable option for LLM usage

  You should be able to flip a USE_AI or somesuch bit without the need to modify how you're calling into osrlib. Depending on the enabled/disabled flag, the code path would just either call the OpenAI API to get strings or use those provided by the developer/adventure designer.

• Document how to enable/disable said LLM switch in the top-level README as well as the osrlib README.

  The top-level README currently makes it sound like the easy switchyswitchy is already in, and while you can enable/disable it right now, it's not clear how (and it's not clean as described above).