feat(skills): add style review workflow

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

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+# Let rules
+
+## Prefer explicit dependencies over shared convenience bindings
+
+### Rule
+
+Do not add a `let` binding only to let sibling values share one expression.
+Apply this rule only when one sibling value clearly owns the concept.
+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.
+Do not apply this rule when a binding names a local constant used to keep sibling assignments in sync.
+
+### Rationale
+
+Shared convenience bindings hide dependency direction.
+Explicit references show which value owns a concept and which value consumes it.