feat: add red-green-proof debugging skill

[rooagidev]

Summary

• Add the red-green-proof Agent Skill under skills/red-green-proof/SKILL.md.
• Require cause verification before test authoring.
• Require a focused red test, smallest fix, deliberate revert-to-red check, and final full-suite validation.
• Document honest alternatives for inaccessible code paths: extraction, structural, model, and characterization tests.

Why

This provides a disciplined debugging workflow for proving that a regression test is load-bearing rather than merely passing after a fix.

Validation

• git diff --check
• YAML frontmatter follows this repository's skill template with only name and description.
• The contribution is a self-contained Markdown skill with no runtime dependencies.

Provenance and attribution

This skill is contributed by RooAGI and developed in the open at https://github.com/RooAGI/red-green-proof. The source project uses the RooAGI brand and MIT licensing; this upstream contribution intentionally contains only the skill payload so it fits the repository's existing self-contained skill convention.