Diagnostic only, never to be merged. Since #232 no workflow calls the
Actions cache, so nothing has shown whether a job container can reach the
cache server today. This job prints the cache address the runner
advertises (path redacted, since job logs are public), probes it over TCP
next to other ports and host addresses, and round-trips a tiny payload
through actions/cache. It is green only if the payload comes back.
Refs #234
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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