Serve Markdown to AI Agents with Accept Headers

The service adds content-negotiation support so that requests with the header Accept: text/markdown receive a pure Markdown version of the page, while other formats return a 406 error. It sets the Vary: Accept header, respects q-values, and removes navigation, scripts, ads, and layout markup, reducing token count, improving signal-to-noise for retrieval-augmented generation, and lowering latency.