Add staging subdomain

[flakey5]

No description provided.

[cursor]

PR Summary

Low Risk
Infrastructure-only Wrangler routing and config cleanup; no application code changes.

Overview
Adds a staging route so the staging Cloudflare Worker env serves traffic on r2-staging.nodejs.org/* on the nodejs.org zone.

Also fixes workers_dev placement: it is moved out of vars (where it was not a real env binding) to the correct Wrangler fields—workers_dev: true on the default config and staging env, and workers_dev: false on prod.

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[ovflowd]

@flakey5 updates here?