Rules example uses a tag the seed vocabulary lacks
What happened: the frontmatter example in rules/v1.md uses tags: [billing, domain];
billing isn't in scaffold/docs/tags.md. An agent copying the example breaks the vocabulary rule
on its first doc. Principle pages repeat the pattern in their examples.
Cost: one wrong tag per adoption, until a checker exists to flag it.
Idea for a fix: in v2, add a comment # billing: added to tags.md in the same change,
or have check skip fenced code blocks so examples stay illustrative.