fix: remove duplicated headers from blog post bodies

- remove extracted title, byline, date, avatar, and featured image preambles
- prevent both extraction scripts from recreating duplicated headers
- add blog content duplication auditing
- update migration documentation

www/tools/audit-blog-content.mjs

@@ -0,0 +1,31 @@
+import fs from 'node:fs/promises';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import fg from 'fast-glob';
+import matter from 'gray-matter';
+
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const failures = [];
+const files = await fg('src/content/blog/**/*.{md,mdx}', { cwd: root, absolute: true });
+
+for (const file of files) {
+  const { content, data } = matter(await fs.readFile(file, 'utf8'));
+  const body = content.trimStart();
+  if (body.startsWith('# ')) failures.push(`${path.relative(root, file)}: body begins with a duplicate article title`);
+  if (body.includes(data.featuredImage) && body.indexOf(data.featuredImage) < 500) {
+    failures.push(`${path.relative(root, file)}: body begins with a duplicate featured image`);
+  }
+}
+
+const report = [
+  '# Blog Content Audit',
+  '',
+  `Checked ${files.length} blog post bodies for layout content duplicated in Markdown.`,
+  '',
+  failures.length ? failures.map((failure) => `- ${failure}`).join('\n') : 'No duplicated article headers were detected.',
+  ''
+].join('\n');
+
+await fs.writeFile(path.join(root, 'reports/blog-content-audit.md'), report);
+console.log(`${failures.length} duplicated blog header elements.`);
+if (failures.length) process.exitCode = 1;