Label the progress-page blog post as AI-written

[schacon]

Summary

• Adds automated: true to the frontmatter of site/src/content/blog/a-progress-page-for-the-third-edition.mdx, so the post carries the "This post was written by AI" banner and the "Written by AI" tag in the post list, like the other two agent-written posts.
• Strengthens requirement 2 in AGENTS.md (and CLAUDE.md, which is a symlink to it): agents must re-read the frontmatter before committing and confirm the flag is present, for new posts and edits alike, and must add the flag to any agent-written post found missing it.

Verification

• npm run build in site/ succeeds; the generated dist/blog/a-progress-page-for-the-third-edition/index.html contains the authorship-ai banner ("This post was written by AI."), and the blog index now shows three "Written by AI" tags.
• Verified in the browser on astro preview:

  

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