Fetch a URL and return the main content as Markdown.

First tries a plain HTTP request with an Accept: text/markdown header. If the server responds with Content-Type: text/markdown (e.g. Cloudflare Markdown for Agents sites), the body is returned immediately without launching a browser.

Otherwise, uses patchright (a Playwright fork with anti-detection patches) to drive real Chromium, which clears most Cloudflare bot challenges and renders JavaScript-required pages. A single headless Chromium instance is kept alive across calls so subsequent fetches avoid the browser cold-start cost (~2-5s). After navigation, polls the page DOM and runs trafilatura, returning as soon as the extracted Markdown stabilizes across two consecutive polls — typically within a few hundred milliseconds of the DOM being built, regardless of whether trackers, ads, and analytics are still loading in the background.