fix(agents): validate iframe document references

docs/development/agent-workflow.md

@@ -57,6 +57,7 @@ Only headings present outside inert HTML containers contribute slugs or duplicat
 Explicit HTML anchors use `parse5`, excluding comments, scripts, styles and template contents.
 Rendered HTML anchor and image-map area hrefs and image sources use the same local-reference checks
 as Markdown links, including decoded attributes and fragment validation.
+Iframe sources are document references and retain Markdown-target anchor checks.
 Image references check file existence without interpreting image fragments as
 Markdown headings; document links keep anchor checks even when sharing a target.
 HTML video, audio, source and track `src` assets and video posters use the same