v6.2.0

The URL layer stops being a copy. This server used to carry byte-identical
vendored mirrors of the fleet's origin resolver and path builders, kept honest
by a drift guard; it now depends on the published package instead.

Changed

• Depends on @extension.dev/urls instead of vendoring it. The mirrors at
  lib/urls-origins.ts and lib/urls-paths.ts are gone, and registry.ts,
  login-flow.ts, and create.ts import the package directly. It is bundled
  into dist, so the install footprint is unchanged: no new runtime
  dependency, same standalone server.
• preview.extension.dev resolves like every other fleet origin. preview
  is now a first-class origin in the shared resolver, so EXTENSION_DEV_PREVIEW_URL
  is honored and an unset preview host follows the same local-vs-prod signal as
  console, inspect, and registry rather than defaulting to production.