docs: add claude test skill

[margaretjgu]

Test stage is still 0. Adds .claude/skills/test.md so the collector can score Experimenting (level 1).

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	0.5s
✅ REPOSITORY	gitleaks	yes		no	no	52.91s
✅ REPOSITORY	git_diff	yes		no	no	0.09s
✅ REPOSITORY	secretlint	yes		no	no	1.99s
✅ REPOSITORY	trivy	yes		no	no	21.4s

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