Feat: Host the NSA article's header image next to the article

Relative header_image paths (./img.webp) now resolve to /<slug>/img.webp, and article
assets also ship unhashed so og:image and JSON-LD URLs resolve.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/generate-pages.mjs

@@ -63,6 +63,8 @@ const run = async () => {
 
     articles.push({
       ...data,
+      // "./img.webp" points at a file next to index.md, which gets copied to /<slug>/
+      header_image: data.header_image?.replace(/^\.\//, `/${data.slug}/`),
       sourceFile: file,
       markdown: md,
       isUnlisted: unlisted.has(data.slug),