ci: add NixOS module test workflow

Add GitHub Actions workflow to test the NixOS module on changes to
nix files or Go source code. The workflow:

- Checks module syntax with 'nix flake check'
- Builds the NixOS integration test
- Validates module can be imported and evaluated
- Tests example configuration evaluates correctly

Runs on push to main and pull requests, with change detection to
skip when neither nix nor Go files are modified.

.github/workflows/nix-module-test.yml

@@ -0,0 +1,56 @@
+name: NixOS Module Tests
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+concurrency:
+  group: ${{ github.workflow }}-$${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  nix-module-check:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          fetch-depth: 2
+
+      - name: Get changed files
+        id: changed-files
+        uses: dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36 # v3.0.2
+        with:
+          filters: |
+            nix:
+              - 'nix/**'
+              - 'flake.nix'
+              - 'flake.lock'
+            go:
+              - 'go.*'
+              - '**/*.go'
+              - 'cmd/**'
+              - 'hscontrol/**'
+
+      - uses: nixbuild/nix-quick-install-action@889f3180bb5f064ee9e3201428d04ae9e41d54ad # v31
+        if: steps.changed-files.outputs.nix == 'true' || steps.changed-files.outputs.go == 'true'
+
+      - uses: nix-community/cache-nix-action@135667ec418502fa5a3598af6fb9eb733888ce6a # v6.1.3
+        if: steps.changed-files.outputs.nix == 'true' || steps.changed-files.outputs.go == 'true'
+        with:
+          primary-key:
+            nix-${{ runner.os }}-${{ runner.arch }}-${{ hashFiles('**/*.nix',
+            '**/flake.lock') }}
+          restore-prefixes-first-match: nix-${{ runner.os }}-${{ runner.arch }}
+
+      - name: Run NixOS module tests
+        if: steps.changed-files.outputs.nix == 'true' || steps.changed-files.outputs.go == 'true'
+        run: |
+          echo "Running NixOS module integration test..."
+          nix build .#checks.x86_64-linux.headscale -L