Fix: add signing env vars to library build step - #17

Why

The library build step runs ./gradlew build publishToMavenLocal after the version has been changed from SNAPSHOT to a release version (e.g., 6.0.0-rc1). This triggers GPG signing because signAllPublications() is configured in build.gradle and the version no longer ends with -SNAPSHOT.

Without the signing environment variables, the build fails with:

> Task :signMavenPublication FAILED
> Cannot perform signing task ':signMavenPublication' because it has no configured signatory

Changes

• Added signing environment variables to the "Build library" step:
  • ORG_GRADLE_PROJECT_signingInMemoryKey
  • ORG_GRADLE_PROJECT_signingInMemoryKeyPassword

Testing

Verified that:

• ✅ Library build with SNAPSHOT version → signing skipped (works without env vars)
• ❌ Library build with release version → signing required (fails without env vars)
• ✅ Library build with release version + signing env vars → works
• ✅ Gradle plugin build → no signing configured (works without env vars)
• ✅ Example builds → no signing configured (works without env vars)

Note: Removed ORG_GRADLE_PROJECT_signingInMemoryKeyId because the plugin requires it in short 8-character format, and it's optional (auto-detected from the key).