Add engineering-loop skill

[Phelan164]

Summary

Add the external engineering-loop skill to Development & Code Tools.

Why this skill

The skill provides one lifecycle owner for a software change:

1. establish scope and a baseline or reproduction;
2. implement the smallest coherent change;
3. run focused and repository-required checks;
4. review the actual diff with findings first;
5. fix verified issues and report evidence and residual risk.

It uses progressive disclosure through a concise SKILL.md plus loop-contract
and hard-debugging references. The entry links to the original maintained skill
and its supported installer command instead of duplicating its files in this
repository.

The current model-adaptive version is published in
Codex How To v0.3.0.

Verified installation

• catalog: https://skills.sh/phelan164/codex-howto/engineering-loop
• npx skills add Phelan164/codex-howto --skill engineering-loop -g -a codex -y
• isolated Codex installation smoke test: passed

Validation

• official skill-creator/scripts/quick_validate.py: Skill is valid!
• repository validation and GitHub Actions pass on v0.3.0
• verified the current catalog has no engineering-loop entry
• git diff --check