"Vague prose doesn't change agent behavior any more than it changes a new hire's behavior on day one" is exactly right, and the comparison holds further than it first looks. What actually changes a new hire is a failing test, a linter and a review that blocks, not the onboarding doc. Same here: the rules that survive are the ones something else enforces.

The pattern that has worked best for me is treating the file as a pointer to enforcement rather than a substitute for it. State the rule, then state which check fails when it is broken.

Anything in the file with no corresponding check is a preference, and marking it as one keeps the genuinely load-bearing lines from being diluted.