docs(skills): add nix let review rule

src/skills/nix-style/rules/let.md

@@ -0,0 +1,16 @@
+# Let rules
+
+## Prefer explicit dependencies over shared convenience bindings
+
+### Rule
+
+Do not add a `let` binding only to let sibling values share one expression.
+When one sibling value owns the concept, reference that sibling value directly.
+When a flake output owns the public value, reference that output through `self`.
+Do not make an output attrset recursive only to reference sibling outputs.
+
+### Rationale
+
+Shared convenience bindings hide dependency direction.
+Explicit references show which value owns a concept and which value consumes it.
+