add celements-velocity skill

[cbspichler]

Summary

Adds a shared celements-velocity agent skill for maintaining legacy Apache Velocity 1.7 templates in Celements and Celements-based Progon projects.

The skill provides conservative, Celements-specific guidance:

• Treat Velocity as legacy and prefer established alternatives where practical.
• Use Apache Velocity 1.7 documentation and avoid current XWiki documentation.
• Keep substantial Velocity code in version-controlled .vm files and integrate it using #parse.
• Prefer verified Celements ScriptServices over introducing new $xwiki usage.
• Encourage quiet references for nullable output.
• Keep templates focused and move substantial logic to Java where practical.
• Reuse existing Celements/frontend components and the Celements JSON builder.

Also adds the skill to skills/README.md so it is included in the shared skill collection documentation.

[cbspichler]

I was bored with telling my agents the same things over and over again. I tried to keep it basic, so as not to claim wrong things. After all my own knowledge about Velocity in Celements is not exactly large. It's AI-generated, so please review carefully and add/delete as necessary.