feat(opencode): prioritize text/markdown in webfetch accept header

[zeke]

Resolves: #13486

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR adds text/markdown as the highest priority in the default Accept header for the webfetch tool. When a server supports content negotiation and can serve markdown directly, this avoids unnecessary HTML-to-markdown conversion on the client side.

Cloudflare recently launched Markdown for Agents, which uses the Accept: text/markdown header to serve markdown versions of web pages at the edge. Any Cloudflare zone with this feature enabled will return markdown instead of HTML when the request includes text/markdown in the Accept header. By prioritizing this in the webfetch tool, opencode gets cleaner output with fewer tokens from any site that supports it.

How did you verify your code works?

Added tests covering accept header behavior for each format option and verifying markdown passthrough vs HTML conversion. Ran the test suite in packages/opencode.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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