AI Scaffolding: Add .agents skill for add-training-runtime

### What you would like to be added?

Add an `.agents/skills/add-training-runtime/SKILL.md` skill file following the common `.agents` format (matching the pattern from [kubernetes-sigs/jobset#1272](https://github.com/kubernetes-sigs/jobset/pull/1272)).

This skill provides a step-by-step guide for adding a new training runtime plugin to the Kubeflow Trainer extension framework, including plugin implementation, factory registration, and integration with the runtime chain.

### Why is this needed?

AI agents and new contributors currently reverse-engineer patterns from code, which produces inconsistent implementations and missed steps (forgetting plugin registration, incorrect Info object usage, or missing ML policy handling). A checked-in skill guide ensures consistent, correct plugin scaffolding across tools and contributors.

Supersedes #3713.