docs: require system environment variables when deploying to Vercel (#4166)

* docs: require system environment variables when deploying to Vercel

Workflow recognizes a Vercel deployment by VERCEL_DEPLOYMENT_ID, which
Vercel only exposes when "Enable access to System Environment Variables"
is selected in the project's Environment Variables settings. A project
that cleared that checkbox looks like a non-Vercel environment to the
SDK: the build emits local output and the runtime selects the Local
World, whose writes then fail on the deployment's read-only filesystem.
Nothing announces the wrong World, because the "local world inside a
Vercel deployment" warning keys off the same missing variable.

Document the setting as a deployment prerequisite on both deploying
pages and both Vercel World pages, add a diagnosis section covering the
symptoms and why WORKFLOW_TARGET_WORLD=vercel is not a workaround, and
note it alongside World selection in the v5 configuration reference.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Co-Authored-By: Nathan Rajlich <71256+TooTallNate@users.noreply.github.com>

* docs: qualify default World selection warnings

Co-Authored-By: Nathan Rajlich <71256+TooTallNate@users.noreply.github.com>

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Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
Co-authored-by: Nathan Rajlich <71256+TooTallNate@users.noreply.github.com>