[PR Preview] Integrate extension workflow with freighter-config (sandbox URLs)

[piyalbasu]

Goal

Update .github/workflows/prPreview.yml to fetch the PR author's sandbox URLs from freighter-config at build time, replacing the staging URLs used in Phase 1.

Acceptance criteria

• PR opened by an engineer with a freighter-config entry builds against their sandbox URLs (verified by inspecting the built bundle)
• PR opened by an engineer without a freighter-config entry falls back to dev/staging URLs and posts a bot comment on the PR explaining why
• If freighter-config is unreachable at build time, falls back to last-known cached URLs and labels the PR preview-degraded
• Documented in the stakeholder install guide that sandbox-backed previews require sshuttle (with link to install guide)

Dependencies

• Blocks on [PR Preview] Create freighter-config repo for engineer → sandbox URL map #2765 (freighter-config repo creation)

Reference

Fullstack PR Preview Flow design doc — § Pathways to Achieving Fullstack PR Previews → Extension, § Failure Modes