feat(review): PR context panel — summary, comments, and checks

[backnotprop]

Summary

• Adds Summary, Comments, and Checks tabs to the ReviewPanel when reviewing a GitHub PR
• Tabs appear as icon buttons inline in the panel header (only in PR mode, local review unchanged)
• Data fetched lazily via new /api/pr-context endpoint on first tab click
• Uses DOMPurify for safe rendering of GitHub markdown/HTML content in comments and PR body
• Extended inline markdown renderer with link and bare URL support

Partial progress on #348 (PR Context Panel section complete).

Details

Backend:

• PRContext type + fetchPRContext() with defensive parsing of gh pr view --json response (handles missing fields, nested author objects, varied shapes)
• New GET /api/pr-context endpoint in review server (PR mode only)

Frontend:

• usePRContext hook — lazy fetch with loading/error states
• PRSummaryTab — PR body (block-level markdown), labels (colored badges), linked issues, state/draft badge
• PRCommentsTab — chronologically merged comments + reviews with state badges
• PRChecksTab — merge readiness (handles merged/closed/open states), CI checks grouped by workflow
• MarkdownBody shared component with DOMPurify sanitization for HTML passthrough

Test plan

• plannotator review https://github.com/owner/repo/pull/N — tabs appear in panel header
• Click Summary → PR description, labels, linked issues render
• Click Comments → chronological thread with review state badges
• Click Checks → merge status + CI checks with pass/fail icons
• Merged PRs show "Merged" badge (not "Unknown")
• Local review (plannotator review) → no tabs, unchanged behavior
• HTML in comments (e.g. <sub>) renders safely via DOMPurify
• bun test packages/shared/ && bun test packages/review-editor/ — all tests pass

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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