New Experiment: Markdown feeds (powered by `html-to-md`)

What problem does this address?

Markdown output can make content easier for AI tools, agents, search/indexing workflows, static exports, and reader experiences that prefer structured plaintext.

The prior implementation in #194 explored much of the product surface, including Markdown feed output, singular Markdown views, autodiscovery links, Accept: text/markdown support, settings toggles, filters, and tests. This new experiment should revisit that direction with https://github.com/dmsnell/html-to-md as the conversion layer rather than maintaining a separate converter.

What is your proposed solution?

Create a new Markdown Feeds experiment that exposes content as Markdown, building on the prior work in #194 and using https://github.com/dmsnell/html-to-md for HTML-to-Markdown conversion.

Potential scope to consider:

• Add a new Markdown Feeds experiment, disabled by default
• Provide Markdown output for feed requests
• Support Markdown output for singular content
• Prefer ?format=md as the primary URL strategy
• Keep .md URL support optional (likely via an experiment option that's hidden by default behind the work being done in #842), given prior concerns around WordPress permalink structure
• Consider Accept: text/markdown negotiation as an optional setting
• Use dmsnell/html-to-md for conversion
• Include filters for modifying or extending feed and singular Markdown sections
• Add tests covering feed output, singular output, visibility rules, URL handling, and conversion behavior

Why is this important to you?

No response

I have searched existing issues and this is not a duplicate.

• Yes