owner-led(visual-agent): R2 image hosting + managed PR comment

[JSONbored]

Parent roadmap: #525
Parent epic: #577

Owner-led — assigned to @JSONbored.

Goal

R2 image hosting + the managed PR comment: upload before/after/diff PNGs to R2 with public URLs, then upsert a single marker comment embedding them in a per-route before/after/diff table.

Reuse

R2 usage + upsertMarkerComment idempotency from the submission-gate pattern.

Scope

• Upload images to R2 under a PR/SHA key; public (or signed) URLs; lifecycle/expiry.
• Compose the managed comment (per-route rows: before | after | diff | change %); idempotent upsert keyed by REVIEW_MARKER.
• Supersede stale comments on new pushes.

Acceptance

A visual PR gets exactly one managed comment with embedded before/after/diff images that updates in place on new pushes.

[JSONbored]

Closing as not planned (2026-07-01 issue audit): R2 hosting + managed-comment embedding delivered by converged engine (shot.ts R2 + unified-comment.ts, #1490).