feat(skills): Add Nexus package creation skill

[christian-pinto]

This PR adds a skill for supporting users in creating a new PR ti add a Nexus Package to the project.

The SKILL helps with:

• Identify the python package, models and vLLM dependencies
• Identify the target variant
• Add the python package to the Nexus dependencies
• Create and validate a Nexus Package metadata folder
• Open PR / Create PR text

This pr closes #40 and #24