Add Markdown Feeds experiment

[Jameswlepage]

Summary

Adds Markdown representations of WordPress content for feeds and singular views:

• Feed at /?feed=markdown with autodiscovery links
• Singular Markdown via .md permalinks and Accept: text/markdown
• HTML to Markdown conversion via WP_HTML_Processor with fallback to WP_HTML_Tag_Processor
• Section-level filters to reorder/extend output (e.g., inject custom fields)
• Integration tests for feed and converter behavior

Implementation Details

• Markdown_Feeds registers the feed, .md parsing, and Accept header negotiation
• HTML_To_Markdown_Converter uses the WordPress HTML API to produce Markdown
• Settings toggles for feed, .md permalinks, and Accept header negotiation
• New filters: ai_experiments_markdown_feed_post_sections and ai_experiments_markdown_singular_post_sections

Test plan

• Enable experiment under Settings -> AI Experiments
• Visit /?feed=markdown and confirm text/markdown output
• Visit /hello-world.md and confirm text/markdown output
• Request a post with Accept: text/markdown and confirm Markdown output
• Optional: use the new section filters to inject custom fields into output
• npm run test:php -- --filter Markdown_Feeds

[codecov]

Codecov Report

❌ Patch coverage is 40.51447% with 370 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.62%. Comparing base (1351969) to head (50ae614).
⚠️ Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
...udes/Experiments/Markdown_Feeds/Markdown_Feeds.php	22.91%	185 Missing ⚠️
...eriments/Markdown_Feeds/Markdown_Feed_Renderer.php	0.00%	94 Missing ⚠️
...ents/Markdown_Feeds/Markdown_Singular_Renderer.php	0.00%	71 Missing ⚠️
...ents/Markdown_Feeds/HTML_To_Markdown_Converter.php	90.74%	20 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #194      +/-   ##
=============================================
- Coverage      50.80%   49.62%   -1.19%     
- Complexity       375      610     +235     
=============================================
  Files             27       31       +4     
  Lines           1978     2827     +849     
=============================================
+ Hits            1005     1403     +398     
- Misses           973     1424     +451     

Flag	Coverage Δ
unit	49.62% <40.51%> (-1.19%)	⬇️

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[Jameswlepage]

Marking this as an open PR so we can get this merged. @m and others are quite interested in this feature

[github-actions]

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Co-authored-by: Jameswlepage <isotropic@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: m <matt@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[m]

We have some pretty URL prior art here with /feed/[format] that we should support in addition to the query string.

This would be the first time we introduce dots into WP's canonical URL structures... how should we think about that? Remember that we have other things that exist in the hierarchy of the permalink itself, so when you make the permalink a filename you lose that ability to have sub-directories or files.

[jeffpaul]

@Jameswlepage before we move for review, I think Matt raised some questions that we should settle out before proceeding. Did you have immediate reactions to those questions (especially in case you've had conversations on this topic that would inform approach here)? If not, then I can look to sort through those with Darin to get this PR back on track towards review.

[Jameswlepage]

Ah shoot, I didn't actually push up my changes. Yeah, I agree with Matt on the file extension. I'm seeing it as a common convention, so my inclination is to keep it but default to a format query parameter instead of the extension by default. I am not on my desktop right now, so I'll see if I can get that up soon.