feat(skills): add nix style guidance

src/skills/nix-style/RULE-FORMAT.md

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+# Rule format
+
+Use this structure for each rule:
+
+```md
+## Rule title
+
+### Rule
+
+State enforceable instructions.
+
+### Rationale
+
+Explain why the rule exists.
+```
+
+Add `### Examples` only when examples clarify the rule.

src/skills/nix-style/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.
+
+## Load rules
+
+Read every Markdown file under [`rules/`](rules/) before applying this skill.
+Discover rule files from the skill source:
+
+```bash
+find src/skills/nix-style/rules -maxdepth 1 -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.
+When loaded rules conflict, follow the most specific rule.
+
+After editing Nix files, run:
+
+```bash
+nix run .#fix
+```
+
+Finish when the Nix change follows the loaded rules.
+
+## Maintain rules
+
+When adding or changing files under [`rules/`](rules/), read [`RULE-FORMAT.md`](RULE-FORMAT.md) first.
+Finish when the changed rule files follow that format.

src/skills/nix-style/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.