docs(skills): surface nix rule format

src/skills/nix-style/SKILL.md

@@ -18,6 +18,11 @@ find src/skills/nix-style/rules -maxdepth 1 -type f -name '*.md' -print | sort
 
 Finish when all discovered rule files have been read.
 
+## Maintain rules
+
+When adding or changing files under [`rules/`](rules/), read [`RULE-FORMAT.md`](RULE-FORMAT.md) first.
+Finish when the changed rule files follow that format.
+
 ## Apply rules
 
 Apply the loaded rules when editing or reviewing Nix files.