Linux users cannot update because COS has no linux-x64/linux-arm64
binaries — the CI matrix and npm publish pipeline had Linux commented
out. Re-enable ubuntu-latest and ubuntu-24.04-arm runners, add
linux-x64/linux-arm64 to npm PLATFORMS array, SUPPORTED_PACKAGES set,
and optionalDependencies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>