feat: add instruction for claude hooks

.claude/test-runner.md

@@ -0,0 +1,34 @@
+# Test Runner Instructions
+
+## Determine affected hosts
+
+Run `git diff --name-only HEAD` (or `git diff --name-only` for unstaged changes) to find changed files.
+
+### Scope rules
+
+- If changes are **only** under `hosts/<name>/`, test only that specific host:
+  ```bash
+  ./scripts/test-configuration <name>
+  ```
+- If changes touch `utils/`, root-level `.nix` files, or any shared configuration, test **all** hosts:
+  ```bash
+  for host in amzebs-01 dev fw mail nas nb web-arm; do
+    ./scripts/test-configuration "$host"
+  done
+  ```
+
+### Custom package changes
+
+If any files under `utils/pkgs/<package-name>/` were modified, also build the package directly:
+
+```bash
+nix-build -E 'with import <nixpkgs> { overlays = [ (import ./utils/overlays/packages.nix) ]; config.allowUnfree = true; }; <package-name>'
+```
+
+This catches build failures that `test-configuration` (evaluation-only) would miss.
+
+## Notes
+
+- Each `test-configuration` run performs a `nix-instantiate` dry-build (evaluation only, no binary builds).
+- A non-zero exit code from any host means the test failed.
+- Hosts: `amzebs-01`, `dev`, `fw`, `mail`, `nas`, `nb`, `web-arm`.