Allow LLM-based help search chat

There are oodles and oodles of information locked in the Amplenote Help Center. The search functionality is pretty good if you know exactly what to look for, but it would be a lot better if it had a GitClear-style agent that could browse indexed pages and reason about the best answer to the user's question.

Since this agent already lives in GitClear, porting it to Amplenote should not be more than a day or two of work.