feat(skills): add nix style guidance

src/skills/nix-style/SKILL.md

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+---
+name: nix-style
+description: Repository Nix style, architecture, and check conventions. Use when editing or reviewing Nix files in this repository.
+---
+
+# nix-style
+
+Use this skill as the source of truth for repository Nix conventions.
+
+## Load rules
+
+Read every Markdown file under [`rules/`](rules/) before applying this skill.
+Use this command from the skill directory when file names are unknown:
+
+```bash
+find rules -type f -name '*.md' -print | sort
+```
+
+Finish when all discovered rule files have been read.
+
+## Apply rules
+
+Apply the loaded rules when editing or reviewing Nix files.
+Prefer the most specific rule when two rules cover the same code.
+
+After editing Nix files, run:
+
+```bash
+nix run .#fix
+```
+
+Finish when the Nix change follows the loaded rules.

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

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+# Flake rules
+
+## Keep `flake.nix` as flake wiring
+
+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.
+
+This keeps dependency declarations separate from output logic.