docs(skills): separate nix rule format

src/skills/nix-style/RULE-FORMAT.md

@@ -0,0 +1,17 @@
+# Rule format
+
+Use this structure for each rule:
+
+```md
+## Rule title
+
+### Rule
+
+State enforceable instructions.
+
+### Rationale
+
+Explain why the rule exists.
+```
+
+Add `### Examples` only when examples clarify the rule.

src/skills/nix-style/SKILL.md

@@ -18,24 +18,6 @@ find src/skills/nix-style/rules -maxdepth 1 -type f -name '*.md' -print | sort
 
 Finish when all discovered rule files have been read.
 
-## Write rules
-
-Use this structure for each rule:
-
-```md
-## Rule title
-
-### Rule
-
-State enforceable instructions.
-
-### Rationale
-
-Explain why the rule exists.
-```
-
-Add `### Examples` only when examples clarify the rule.
-
 ## Apply rules
 
 Apply the loaded rules when editing or reviewing Nix files.