Tell an agent what a rule in this repository is
An agent asked to "add a rule" in this repository reaches for a style guide,
because that is what the phrase means nearly everywhere else. Here a rule is a
`Rule` value with a check, a registry entry and test resources behind it, and
prose in a file enforces nothing.
`CLAUDE.md` is loaded before the first tool call, which is the part that
matters: the failure it is aimed at is not misreading `README.md` but never
opening it.
It also writes down the feedback loops and the two things that are easy to get
wrong here: the commands run in the dev shell, and `nix flake check` reads the
git tree, so a new file has to be added before it is seen.
No code changes.