feat: centralize npm build workflow

[vitormattos]

Summary

Continue #15 by replacing the deprecated Nextcloud node.yml path with the current npm-build.yml model.

Nextcloud's current node.yml is intentionally a failing no-op that asks consumers to migrate to npm-build.yml and is scheduled for removal at the end of 2026.

This PR adds:

• reusable .github/workflows/npm-build.yml;
• thin workflow-templates/npm-build.yml caller;
• organization catalog metadata.

The caller is pinned to immutable commit bab6f01a48474e6b00feb71882be2f4dafe928fc.

LibreCode adaptations

Compared with the current Nextcloud npm-build.yml:

• replace ubuntu-latest-low with public ubuntu-latest;
• keep actions pinned to full SHAs;
• select Node from package engines with the current upstream fallback;
• use the npm bundled with that Node runtime and enable engine-strict, avoiding installation of a PR-controlled npm range as a global package;
• execute the pinned lockfile repair/check helper through npm exec instead of installing it globally;
• simplify the dirty-working-tree check without nested bash -c;
• pass summary state through environment variables;
• preserve the final check name node so existing branch protection checks keep matching.

Consumer migration

For consumers such as LibreCodeCoop/extract, the migration should rename the installed workflow from node.yml to npm-build.yml, while the reusable implementation keeps the protected summary check name node.