feat(skills): add nix style guidance

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.