NLWeb /ask: a natural-language question, answered as matching items

Implements the NLWeb protocol (github.com/nlweb-ai/NLWeb) in list mode: keyword retrieval over the build log, docs links and site pages, each result with its schema.org object. summarize and generate are answered as lists and say so in _meta. Streams as Server-Sent Events (start, result, complete) when streaming=true, prefer.streaming=true, or Accept: text/event-stream; plain JSON otherwise. POST accepts the same parameters as a JSON body.