feat(skills): add style review workflow

src/skills/style/nix/SKILL.md

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+---
+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.

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.