feat(skills): add style review workflow

src/skills/style/nix/SKILL.md

@@ -0,0 +1,18 @@
+---
+name: nix-style
+description: Use when creating, editing, or reviewing Nix files in this repository.
+---
+
+# nix-style
+
+Use this skill as the source of truth for repository Nix conventions.
+Follow the shared style skill instructions in `src/skills/style/STYLE.md`.
+
+After editing Nix files, run:
+
+```bash
+nix run .#fix
+```
+
+Finish when `nix run .#fix` succeeds and the Nix change follows the loaded rules.
+If the command fails, report the failure.