[PR Preview] Create freighter-config repo for engineer → sandbox URL map

[piyalbasu]

Goal

Create a new stellar/freighter-config repo holding a versioned JSON/YAML map of engineer → sandbox URLs. CI workflows in stellar/freighter and stellar/freighter-mobile will fetch this at PR-preview build time.

Schema (from design doc)

{
  "piyal": {
    "v1": "https://v1-piyal.freighter-sandboxes.kube001-dev.services.stellar-ops.com",
    "v2": "https://v2-piyal.freighter-sandboxes.kube001-dev.services.stellar-ops.com",
    "wb": "https://wb-piyal.freighter-sandboxes.kube001-dev.services.stellar-ops.com"
  }
}

Acceptance criteria

• stellar/freighter-config exists with the schema documented in README
• Restricted access + branch protection on main + required PR approval (because sandbox URLs are private)
• One real engineer entry committed (proves end-to-end)
• CI in the PR Preview workflow can fetch and parse it (verified by the workflow-integration issue)
• Decide whether automatic teardown after N days of inactivity is needed (open question from design doc)
• Onboarding doc: how a new SDF engineer adds their entry (and triggers manage-sandbox action)

Dependencies

• Ops provisions ArgoCD ApplicationSets + wildcard DNS first (*.freighter-sandboxes.kube001-dev.services.stellar-ops.com)

Reference

Fullstack PR Preview Flow design doc — § Dev Environments Per Engineer, § Pathways to Achieving Fullstack PR Previews