feat: add CI workflow and integration test stub

.gitea/workflows/build-nixos.yml

@@ -0,0 +1,41 @@
+name: Build and test NixOS config
+on:
+  pull_request:
+    branches: [ master ]
+    paths:
+      - '**.nix'
+      - 'flake.lock'
+      - 'secrets/**'
+      - 'hosts/**'
+      - 'modules/**'
+  push:
+    branches: [ master ]
+    paths:
+      - '**.nix'
+      - 'flake.lock'
+      - 'secrets/**'
+      - 'hosts/**'
+      - 'modules/**'
+
+jobs:
+  build:
+    runs-on: nixos-builder
+    steps:
+      - name: Checkout
+        run: |
+          git clone -b "${{ github.head_ref || github.ref_name }}" \
+            https://gitea:${{ secrets.GITHUB_TOKEN }}@code.lazyworkhorse.net/gortium/infra.git .
+          git log --oneline -3
+
+      - name: Build NixOS config
+        run: |
+          nix --version
+          nh os build .#lazyworkhorse 2>&1
+
+      - name: Run integration tests (staging VM)
+        run: |
+          echo "==> Deploying PR config to staging VM..."
+          # TODO: pr-test-vm build && pr-test-vm start
+          # TODO: scp test suite to VM, docker compose up, run tests
+          # TODO: pr-test-vm destroy
+          echo "Staging VM integration tests not yet implemented."