Intent: annotate PR comments (Phase 2)

Add an "Annotate" action to every card in the PR comments timeline so a reviewer can attach a note to a comment, have it show in the Annotations sidebar, and ship it to the agent with the comment quoted — the same feedback lifecycle as a diff comment or a description note.

Why. The comments timeline is read-only. Reviewers want to respond to a specific comment — "this is the real concern," "disagree, here's why" — and fold that into the feedback they send the agent, without leaving the review. Unlike code (the agent can read the repo) a PR comment isn't visible to the agent, so the note has to carry the comment with it.

How. Put a single "Annotate" button in the shared hover action row (PRCommentLinkActions), which every card type already uses — so one change surfaces it on comments, reviews, and threads alike. Clicking it opens the existing shared CommentPopover (with Ask AI) anchored to the button; the note attaches to the whole comment (comment-only, no text-selection — comments are short, the trimmed renderer isn't annotation-ready, and selection would fight the card's click/collapse). Keep a simple new CommentAnnotation store ({ id, commentId, commentAuthor, commentBody, text, createdAt }) in App.tsx, threaded through ReviewStateContext; PRCommentsTab reads the handlers from useReviewState() and owns the popover state, threading an onAnnotate callback down to the inline cards and into ThreadCard. Surface the notes in the sidebar under a "PR comments" group (author + comment snippet + the note, select/delete) mirroring the description group, and fold them into totalAnnotationCount and the sidebar count. Wire Ask AI to the existing file-less scope-selection ask (label: 'PR comment', text: comment body). Export via a new dedicated formatter exportCommentAnnotations — author + the full quoted comment body + the note — appended to feedbackMarkdown; deliberately not exportMessageAnnotations, which preflight showed is built for annotating assistant AI messages and would mislabel PR comments to the agent. No server, endpoint, or Pi-runtime changes; work is contained to packages/review-editor plus the shared CommentAnnotation type.

Decision trail:

• ADR: adr/decisions/004-annotate-pr-description-and-comments-20260630-155000.md (Decision 7's exportMessageAnnotations-for-comments is superseded — see spec)
• Spec (greenlit, with preflight findings): adr/specs/comment-annotation-phase2-20260630-193000.md
• Phase 1 (reference): adr/specs/description-annotation-phase1-20260630-171500.md, adr/intent-description-annotation-phase1-20260630-180000.md