Fix Deployment: skip signing snapshots + normalize GPG key

[mustafaozhan]

The Deployment workflow failed on develop with Could not read PGP secret key while signing the snapshot publication.

• Snapshots no longer get signed — Central Portal snapshots don't require signatures, so signAllPublications() is now gated to non-SNAPSHOT versions. The develop publish needs no signing key.
• release.yml normalizes GPG_KEY through printf '%b', so a secret stored with literal \n becomes real newlines for master release signing (a key that already has real newlines passes through unchanged).

Resolves #282