owner-led(visual-agent): base/head render harness (reuse ui:preview)

[JSONbored]

Parent roadmap: #525
Parent epic: #577

Owner-led — assigned to @JSONbored.

Goal

Base/head render harness: serve the UI at the PR's base ref and head ref so both can be screenshotted.

Reuse

gittensory's existing npm run ui:preview (builds the UI + serves via wrangler).

Scope

• Build + serve the UI at two refs (base, head) — either two ephemeral servers or sequential checkouts.
• Health-wait until each server is ready.
• Parameterize the base URL for the capture step.

Acceptance

Both base and head of a PR can be served locally/ephemerally and reached by a browser.

[JSONbored]

Closing as not planned (2026-07-01 issue audit): Base/head render replaced by CF Browser-binding + preview-session capture in the converged engine (#853).