Why (round-5 review, unsupportedpastels m5, D4): `npm install --prefix <cache>` makes the
cache dir the npm project, so the repo's .npmrc (min-release-age=14) was ignored: `npm config
get min-release-age` from the prefix was undefined. The repo .npmrc is now copied into the
prefix before installing (verified: `npm config get min-release-age --prefix <cache>` -> 14).
A failed or hung install raised CalledProcessError/TimeoutExpired, which cli.main does not
catch, so the run exited 1 with a traceback that read like a finding; both now raise
RuntimeError with npm's stderr, which cli.main maps to exit 2.
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