fix(assembly): @unresolved is a provenance comment, not visible output
The unresolved-issues block was rendered as a visible <aside>, so end users
saw an "Unresolved issues" section appended to their document (common at 1
review pass). Render it as an HTML comment like the other @-markers instead;
it stays in the document for tooling and persists in unresolved.md, but no
longer appears in the rendered preview.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>