ci: add nix build-and-deploy GitHub Actions workflow [skip ci]

Adds the canonical Nix CI/CD deploy workflow (install Nix, build .#default,
copy to VPS via SSH, update profile, systemctl restart) previously missing
from this repo. Enforces push-to-GitHub + GHA deploy only (no direct deploy).

.github/workflows/deploy.yml

@@ -0,0 +1,61 @@
+name: Build & Deploy (Nix)
+
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+concurrency:
+  group: deploy
+  cancel-in-progress: false
+
+permissions:
+  contents: read
+
+env:
+  VPS_HOST: ${{ secrets.VPS_HOST }}
+  VPS_USER: ${{ secrets.VPS_USER }}
+
+jobs:
+  build-and-deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v22
+        with:
+          determinate: false
+          extra-conf: |
+            sandbox = false
+            accept-flake-config = true
+
+      - name: Cache Nix
+        uses: DeterminateSystems/magic-nix-cache-action@v14
+
+      - name: Build zesdex
+        id: build
+        run: |
+          nix build .#default --impure --option sandbox false --print-build-logs
+          STORE_PATH=$(readlink result)
+          echo "store-path=$STORE_PATH" >> "$GITHUB_OUTPUT"
+          echo "Build OK: $STORE_PATH"
+
+      - name: Setup SSH key
+        env:
+          SSH_KEY: ${{ secrets.SSH_PRIVATE_KEY }}
+        run: |
+          mkdir -p ~/.ssh
+          echo "$SSH_KEY" > ~/.ssh/id_ed25519
+          chmod 600 ~/.ssh/id_ed25519
+          sed -i 's/\r$//' ~/.ssh/id_ed25519
+          ssh-keygen -y -f ~/.ssh/id_ed25519 >/dev/null 2>&1 || { echo "SSH key invalid"; exit 1; }
+          ssh-keyscan -H "$VPS_HOST" >> ~/.ssh/known_hosts 2>/dev/null
+
+      - name: Deploy zesdex to VPS
+        run: |
+          STORE_PATH="${{ steps.build.outputs.store-path }}"
+          ssh "$VPS_USER@$VPS_HOST" "nix copy --to file:///nix/store $STORE_PATH && nix-env --install --force $STORE_PATH --profile /nix/var/nix/profiles/zesdex && systemctl restart zesdex"