Serve Markdown when a client sends Accept: text/markdown #575

Summary

An agent that asks a page for Markdown with the Accept header gets HTML today. Serve Markdown for that request, and tell clients where the Markdown copy is.

Context

At docs commit aeb5162, GET /overview/ with Accept: text/markdown answers 200 with content-type: text/html. /overview.md answers text/markdown. A path without a trailing slash answers 307 to the slash form. Cloudflare reports that Markdown negotiation works on 3.9% of the top 200,000 domains and names Claude Code, OpenCode and Cursor as clients that send the header by default (https://blog.cloudflare.com/agent-readiness/, April 2026; I did not test those clients).

Acceptance criteria

• A request to any page address that prefers text/markdown in Accept, with or without a trailing slash, answers the same body as the page's .md copy, with content-type: text/markdown and Vary: Accept.
• A browser request still gets HTML, and Vary: Accept stops a cache serving one form to the other.
• Each HTML page has <link rel="alternate" type="text/markdown" href="<path>.md"> in its head.
• A test covers the Markdown request, the HTML request and the missing page (404).

Out of scope

• Generating a skill.md or other agent files.

Notes

Pages are prerendered and AGENTS.md ("Worker route frozen") warns about routes that must stay Worker-served. I did not check whether a static HTML file is served before the Worker runs, which would stop the header being read. If so, the Worker may need to run first for page paths.