[FEATURE]: Support Cloudflare “Markdown for Agents” docs negotiation (`Accept: text/markdown`)

[kdcokenny]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Cloudflare announced “Markdown for Agents” (2026-02-12):
https://blog.cloudflare.com/markdown-for-agents/
Request: add support in opencode’s docs fetching flow for Accept: text/markdown so agent clients can receive markdown instead of HTML when available.
Expected behavior:

• Prefer markdown responses when supported
• Keep current behavior unchanged when markdown is unavailable
• Validate response handling for Content-Type and Vary: Accept
  Somewhat related: [FEATURE]: provide llms.txt and docs as markdown #8816

[kdcokenny]

If this is something y'all are interested in lmk and I'll take it up!

[findalexli]

Yes please!

[zeke]

PR for this: #15899

[zeke]

Re: Vary: Accept — this is a server-side caching directive that tells intermediaries (CDNs, proxies) to cache different responses based on the Accept header. It doesn't require any client-side handling, so I don't think the PR needs to address it.