owner-led: visual-change PR auto-review (before/after screenshots)

[JSONbored]

Parent roadmap: #525

Owner-led tooling — assigned to @JSONbored. Not a Gittensor-scored contributor task.

Goal

Make owner-led visual PRs reviewable at a glance: automatically capture before/after screenshots (plus a pixel diff) on any PR that changes the UI, and post them in a single managed PR comment. This removes the "contributors don't supply visual evidence / visual review is slow" pain and gives a maintainer (or a future review agent) something concrete to reason over.

Built as a separate private Cloudflare Worker mirroring the existing submission-gate maintainer-agent pattern (webhook → queue → review → idempotent marker-comment) used in metagraphed and awesome-claude — kept private so the gating logic isn't gameable.

Approach (heavy reuse)

• Worker skeleton, D1 review-state, R2 audit, queue + DLQ, marker-comment idempotency (upsertMarkerComment), GitHub App token helper (githubJson) — mirror the submission-gate pattern.
• UI rendering — reuse gittensory's npm run ui:preview. Screenshot capture — reuse the existing Playwright smoke harness (scripts/smoke-ui-browser.mjs, test:smoke:browser).
• Image hosting — Cloudflare R2 (already available), no third-party service.

Children (segmented)

• owner-led(visual-agent): worker scaffold + webhook/queue + D1/R2 #578 worker scaffold + webhook/queue + D1/R2
• owner-led(visual-agent): visual-change detection + affected-route mapping #579 visual-change detection + route mapping
• owner-led(visual-agent): base/head render harness (reuse ui:preview) #580 base/head render harness
• owner-led(visual-agent): Playwright before/after capture across viewports #581 Playwright before/after capture
• owner-led(visual-agent): visual diff + change quantification #582 visual diff + quantification
• owner-led(visual-agent): R2 image hosting + managed PR comment #583 R2 hosting + managed PR comment
• owner-led(visual-agent): optional LLM visual review + verdict #584 optional LLM visual review

Done when

• A PR that changes the UI automatically gets one managed comment with before/after/diff screenshots of the affected routes.

Notes

• All children owner-led and assigned to @JSONbored; not for Gittensor contributors. Extension overlays (feat(extension): rebuild as dual-audience overlay (WXT + React) #534) are a separate concern.