Handoff comment renderer

[jlwaugh]

Specify how handoff comments render in the comments / handoff
thread (#31's container).

Cover:

• Structured card showing the parsed JSON metadata:
  • changed_files as a file list with counts
  • verification as a code-formatted list
  • residual_risk as a warning-styled callout (Design system foundations and dark-mode-first approach #29 token)
  • links as clickable cards with URL preview where possible
• Raw JSON expandable for verification — clicking "show raw"
  reveals the underlying ```handoff block as written.
• Fallback rendering when parseHandoff returns null: render
  the raw markdown body of the comment with an "(unparseable
  handoff)" annotation. The handoff format is the v0 spec,
  byte-identical, and evolves additively, so v1 readers must
  accept all v0 handoffs. Handoff comment renderer #34 is the v1-side reader implementing
  this.
• Empty / loading / error states using Design system foundations and dark-mode-first approach #29's canonical patterns.

Uses the v0 parseHandoff library directly — no UI-side parsing
variations.

Soft pattern for import path (decoupling from #2's timing):
#34 may copy the v0 parseHandoff source into the v1 codebase with
a // TODO(#2): replace with import once #2 settles import path
comment. Once #2 closes, replace the copy with the agreed import.
The renderer logic itself is the load-bearing deliverable; the
import-path mechanics are mechanical follow-up.

Cross-track deps: #29 (design system).

Output: docs/ui/0005-handoff-renderer.md + sample card mockup.

Parents

• Design system foundations and dark-mode-first approach #29