Intent: comment on the PR description (Phase 1)

Add the ability to select text in a PR description and leave a comment that behaves like any other review comment — it shows in the Annotations sidebar, counts toward the review, and ships to the agent on Send Feedback.

Why. The PR Overview panel renders the description read-only. Reviewers want to flag specific parts of it — "this claim is wrong," "clarify this" — and have that feedback reach the agent alongside their diff comments, without leaving the review or retyping context. Today there's no way to annotate prose in the code review.

How. Reuse the existing prose-annotation engine (useAnnotationHighlighter + CommentPopover) on the description, which already renders through the shared RenderedMarkdown (so its DOM carries data-block-id and is annotation-ready). Run it comment-only (comment mode → the comment box opens directly on selection; no toolbar, quick-labels, or redline), deliberately simpler than plan/annotate. Wrap the description in a memoized AnnotatableDescription that mounts the hook, renders only CommentPopover, and re-applies highlights on change (idempotent, so safe). Keep a new descriptionAnnotations: Annotation[] store in review-editor/App.tsx, threaded through ReviewStateContext, separate from the diff CodeAnnotation[]. Surface the comments in the existing Annotations sidebar under a "PR description" group via a small new DescriptionAnnotationCard, mirroring the established editorAnnotations render/delete pattern (so select and delete come from the sidebar — no affordance on the highlight). Wire the comment box's Ask AI to the existing file-less scope-selection ask (askAI({ scope: { kind:'selection', label:'PR description', text } })), exactly as the HTML viewer does. Include the prose count in both the app-level totalAnnotationCount and the sidebar's totalCount, and append the notes to feedbackMarkdown via exportAnnotations(parseMarkdownToBlocks(prContext.body), …, 'PR Description Feedback', 'PR description'). Move the .annotation-highlight styles into shared packages/ui/theme.css. No server, endpoint, or Pi-runtime changes; the work is contained to packages/ui (moved CSS) and packages/review-editor (wrapper, store, context, sidebar group, count/export lines).

Research & decision trail:

• ADR: adr/decisions/004-annotate-pr-description-and-comments-20260630-155000.md
• Spec (final, greenlit): adr/specs/description-annotation-phase1-20260630-171500.md
• Synthesis / confidence: adr/research/synthesis-description-annotation-20260630-174500.md
• Renderer research: adr/research/SPIKE-renderer-migration-20260630-155500.md, adr/research/SPIKE-renderer-density-parameterization-20260630-160500.md