Implement package signing

[0xgetz]

Add GPG signing for npm packages to ensure integrity and authenticity.

Objectives

• Set up GPG key infrastructure
• Configure npm to sign packages before publishing
• Add verification instructions for users
• Document the signing process in CONTRIBUTING.md

Security Benefits

• Prevents tampering with published packages
• Verifies package authorship
• Increases user trust

Acceptance Criteria

• All packages are signed with GPG keys
• Signing is automated in the publish workflow
• Verification documentation is available
• CI/CD pipeline includes signing step