Add staging subdomain

[flakey5]

No description provided.

[cursor]

PR Summary

Medium Risk
New public DNS route for staging increases exposure if mis-deployed, but behavior uses the existing staging env; workers_dev move is config-only.

Overview
Adds a staging route so the dist worker is reachable at r2-staging.nodejs.org/* on the nodejs.org zone, alongside the existing staging env settings (ENVIRONMENT: staging, KV, R2).

Wrangler config cleanup: workers_dev is removed from vars and set as a real Wrangler flag—true on the default/dev and staging envs, false on prod—so it no longer leaks into runtime environment bindings.

worker-configuration.d.ts is refreshed (mostly union-type formatting and a Console.assert signature tweak); no application code changes in the diff.

^{Reviewed by Cursor Bugbot for commit 7839cb9. Bugbot is set up for automated code reviews on this repo. Configure here.}

[ovflowd]

@flakey5 updates here?