Missing /api/domains REST endpoint (sandbox 404)

[dligthart]

Summary

The Agent Sandbox UI calls GET /api/domains during context hydration (hydrateSandboxContext()), but this endpoint does not exist in the backend routes, resulting in a 404.

Steps to Reproduce

1. Open the Agent Sandbox UI
2. Observe network tab — GET /api/domains returns 404
3. Backend logs show: Route GET:/api/domains not found

Expected

A GET /api/domains endpoint should return the list of domains for the current authenticated user/key.

Impact

The sandbox cannot auto-detect domain context, forcing users to manually configure domain IDs.

Suggested Fix

Add a GET /api/domains route in src/api/routes.ts that returns domains scoped to the authenticated principal.