feat: add instruction for claude hooks

.claude/lint-fixer.md

@@ -0,0 +1,26 @@
+# Lint Fixer Instructions
+
+## Formatter
+
+This project uses **nixpkgs-fmt** for Nix file formatting.
+
+## How to run
+
+1. Find changed `.nix` files:
+   ```bash
+   git diff --name-only --diff-filter=d HEAD | grep '\.nix$'
+   ```
+2. Format only those files:
+   ```bash
+   nix run nixpkgs#nixpkgs-fmt -- <file1.nix> <file2.nix> ...
+   ```
+3. Stage any formatting changes:
+   ```bash
+   git add <formatted files>
+   ```
+
+## Notes
+
+- Only format files that were actually modified, not the entire repo.
+- There is no `.editorconfig` or other formatter config; `nixpkgs-fmt` uses its own defaults.
+- Non-Nix files do not need formatting.