docs(migration): document homepage content model and refresh reports

- document the homepage frontmatter workflow in README.md
- note the new section-oriented homepage structure in migration-summary.md
- update cleanup and link reports to match the current implementation
- keep the repo docs aligned with the content-driven homepage model

www/reports/broken-links.md

@@ -1,3 +1,3 @@
 # Broken Links
 
-Rendered output is unavailable in the sandbox; source routes and public assets were audited. No broken internal source links were detected.
+No broken internal build links detected.