ci: publish test coverage badge for assert helpers

[tapheret2]

Summary

Actionable follow-up related to: ci: publish test coverage badge for assert helpers.

Why

Improves contributor onboarding and reduces repeated review questions.

Proposal

• Capture the missing guidance in docs (or a short CONTRIBUTING subsection)
• Include at least one concrete example or command
• Link from README if it is discoverability-related

Acceptance criteria

• Doc section exists and is linked
• Example is copy-pasteable
• No unrelated refactors in the implementing PR

Notes

Happy to implement if maintainers want this tracked as a good first issue.