Integrate with Yoast SEO llms.txt to use .md URLs

[ilicfilip]

Summary

• When Yoast SEO's llms.txt feature is active, rewrites post/page URLs to .md versions
• Wraps Yoast's generation triggers (option toggle, settings change, weekly cron) with wpseo_canonical, post_link, page_link, and post_type_link filters at priority 9/11 around Yoast's priority 10
• Only loaded when Yoast SEO is active (WPSEO_VERSION check on plugins_loaded)
• Respects the markdown_alternate_supported_post_types filter — only rewrites URLs for registered post types

Addresses the feature request in #1. The CPT .md URL bug reported in the comment is a separate fix in #3.

Test plan

• Enable Yoast SEO's llms.txt feature
• Verify /llms.txt contains .md URLs for posts and pages
• Verify .md URLs in llms.txt resolve to markdown content (200)
• Verify non-post URLs (categories, sitemap) are unchanged
• Verify products and other unsupported post types are unchanged
• Disable Yoast SEO and verify plugin still works without errors

🤖 Generated with Claude Code

[jdevalk]

Consolidation note: keep this integration PR isolated while URL/rewrite fixes/refactors settle (#8/#9 and RewriteHandler refactors).

[ilicfilip]

I have tested this PR and as far I can see it is working, at least from our end.

While testing this PR, I found that after changing the permalink structure (e.g. to /%postname%/), the URLs in llms.txt become empty, example post:
- [Advanced PHP Techniques](): Advanced PHP tips for modern development

This is a Yoast SEO issue, not caused by our plugin.

Root cause

When the permalink structure changes, Yoast's Indexable_Permalink_Watcher intentionally sets all stored permalinks to NULL in the wp_yoast_indexable table (source) and queues a background re-indexing job.

However, Yoast does not re-trigger llms.txt generation after this happens. The llms.txt file is only regenerated on:

• Weekly cron (wpseo_llms_txt_population)
• Toggling the llms.txt feature in Yoast settings
• Changing llms.txt page selection settings

So if llms.txt is regenerated (e.g. by toggling the feature) before the background re-indexing completes, the indexables still have NULL permalinks, and Canonical_Presenter::get() returns empty strings.

How to reproduce

1. Enable llms.txt in Yoast SEO — URLs are correct
2. Change the permalink structure (Settings → Permalinks)
3. Disable and re-enable llms.txt in Yoast settings
4. Check llms.txt — URLs are empty