[AGENT-2] Contributor skills lack user-facing troubleshooting and experiment-launcher coverage

## Problem

The shipped skills (`review-pr`, `config-conventions`, etc.) are well-designed for contributors but provide no support for users trying to run or debug NeMo-RL. Common usage issues such as hang diagnosis, config validation, and first-run failures are not captured in any skill.

## Suggested fix

Add a user-facing troubleshooting skill and/or an experiment-launcher skill that helps agents guide users through common failure modes and configuration decisions.