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/reports/blog-content-audit.md

@@ -0,0 +1,5 @@
+# Blog Content Audit
+
+Checked 65 blog post bodies for layout content duplicated in Markdown.
+
+No duplicated article headers were detected.