feat(skills): add style review workflow

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

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+# Flake rules
+
+## Keep `flake.nix` as flake wiring
+
+### Rule
+
+Add flake inputs in `flake.nix`.
+Put new flake output implementation in `src/dotfiles/default.nix`.
+Use `flake.nix` to pass inputs into that implementation.
+
+### Rationale
+
+This keeps dependency declarations separate from output logic.

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.