deploy: containerize arcade-eval-ref MCP server + ACR build/push action

Replace the cloudflared quick-tunnel dev pattern with a permanent in-cluster
deployment so the self-hosted Arcade engine reaches the echo/add/whoami reference
server over stable cluster DNS.

- lib/mcp_server/Dockerfile: python:3.12-slim, pip install ., HTTP transport via
  ARCADE_SERVER_{TRANSPORT,HOST,PORT} env overrides (no server.py change needed),
  non-root user, port 8000.
- .github/workflows/build-push-acr.yml: build + push
  servicetitandev.azurecr.io/arcade-eval-ref:1.0.<run_number>. Adapted from
  servicetitan/mem0; needs repo secrets ACR_DEV_USERNAME / ACR_DEV_PASSWORD.
- docs/superpowers/specs design record.

K8s manifests live in k8s-backstage-v2 apps/mcp/arcade-eval-ref/ (separate branch).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/build-push-acr.yml

@@ -0,0 +1,58 @@
+name: Build and Push to ACR
+
+# Builds the arcade-eval reference MCP server image and pushes it to the
+# ServiceTitan dev Azure Container Registry. The image is consumed by
+# apps/mcp/arcade-eval-ref/ in k8s-backstage-v2 (backstage-wus2-v4).
+#
+# Adapted from servicetitan/mem0 .github/workflows/build-push-acr.yml.
+# Requires repo secrets ACR_DEV_USERNAME and ACR_DEV_PASSWORD.
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+    paths:
+      - 'lib/mcp_server/**'
+      - '.github/workflows/build-push-acr.yml'
+
+env:
+  REGISTRY: servicetitandev.azurecr.io
+  IMAGE: arcade-eval-ref
+  VERSION_PREFIX: "1.0"
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to ACR
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ secrets.ACR_DEV_USERNAME }}
+          password: ${{ secrets.ACR_DEV_PASSWORD }}
+
+      - name: Generate image tag
+        id: meta
+        run: |
+          echo "tag=${{ env.VERSION_PREFIX }}.${{ github.run_number }}" >> "$GITHUB_OUTPUT"
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: lib/mcp_server
+          file: lib/mcp_server/Dockerfile
+          push: true
+          tags: |
+            ${{ env.REGISTRY }}/${{ env.IMAGE }}:${{ steps.meta.outputs.tag }}
+          cache-from: type=gha,scope=${{ env.IMAGE }}
+          cache-to: type=gha,mode=max,scope=${{ env.IMAGE }}