Going from a pile of personal CLAUDE.md files and copied Cursor rules to something the whole team shares is a familiar path, and the response here about a commit-time test on the rules file shows where it tends to end up: rules treated as code, with tests, rather than as advice.

On an older codebase like a nine year old Rails app, the rules that earn their place tend to be the boring local ones. Which service objects are legacy and must not be extended, which tests are slow and why, which callbacks have side effects nobody remembers. Those are the same things a new contractor would need on day one.

That makes a useful test for any instruction file: if it would not help a new engineer, it is probably not helping the agent either.