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>

docs/superpowers/specs/2026-06-22-deploy-mcp-to-k8s-design.md

@@ -0,0 +1,73 @@
+# Deploy arcade-eval reference MCP server to backstage k8s
+
+**Date:** 2026-06-22
+**Status:** Approved — implementing
+
+## Goal
+
+Replace the ephemeral cloudflared **quick tunnel** (used to register the
+`arcade-eval-ref` server with the self-hosted Arcade engine) with a permanent
+in-cluster deployment on `backstage-wus2-v4`. The engine then reaches the server
+over stable cluster DNS instead of a `trycloudflare.com` URL that dies on restart.
+
+Relevant eval categories: cat-4 (custom server dev), cat-8 (deployment), cat-9 (DX).
+
+## Architecture / data flow
+
+```
+Arcade engine (ns: arcade)  ──HTTP /worker/*──▶  Service arcade-eval-ref (ns: arcade-eval-ref)
+   registered as type "Arcade"                       └─▶ Deployment: python:3.12 running
+   URI = http://arcade-eval-ref.arcade-eval-ref            mcp_server.server over HTTP :8000
+        .svc.cluster.local:8000                           (echo / add / whoami)
+   Secret = ARCADE_WORKER_SECRET  ◀── same value ──▶  env ARCADE_WORKER_SECRET (SealedSecret)
+```
+
+### Runtime facts (verified by introspecting `arcade-mcp-server` 1.17)
+
+- `app.run()` honors env overrides via `_get_configuration_overrides()`:
+  `ARCADE_SERVER_TRANSPORT=http`, `ARCADE_SERVER_HOST=0.0.0.0`, `ARCADE_SERVER_PORT=8000`.
+  So the hardcoded `127.0.0.1` in `server.py`'s `__main__` is overridden at runtime —
+  **no `server.py` change needed.**
+- `ARCADE_WORKER_SECRET` (settings alias `arcade.server_secret`) → worker routes mount at
+  `/worker/*` (what the engine calls); MCP also served at `/mcp`. FastAPI app, port 8000.
+
+## Components
+
+### 1. `arcade-eval` repo (branch off `main`)
+
+- **`lib/mcp_server/Dockerfile`** — `python:3.12-slim`, `pip install .` (pulls
+  `arcade-mcp-server` + `httpx`), `ENV` transport/host/port, non-root user, `EXPOSE 8000`,
+  `CMD ["python","-m","mcp_server.server"]`.
+- **`.github/workflows/build-push-acr.yml`** — adapted from `servicetitan/mem0`. Pushes
+  `servicetitandev.azurecr.io/arcade-eval-ref:1.0.<run_number>`. Login via repo secrets
+  `ACR_DEV_USERNAME` / `ACR_DEV_PASSWORD`. Triggers: `workflow_dispatch` + push to `main`
+  filtered to `lib/mcp_server/**`.
+
+### 2. `k8s-backstage-v2` repo (branch off `master`)
+
+New dir **`apps/mcp/arcade-eval-ref/`** (Flux's `apps` Kustomization recursively applies
+everything under `apps/`; no per-dir `kustomization.yaml`):
+
+- **`namespace.yaml`** — ns `arcade-eval-ref` (labels per repo convention, `team: infra`).
+- **`server.yaml`** — plain `Deployment` (image
+  `servicetitandev.azurecr.io/arcade-eval-ref:1.0.1`; no imagePullSecret — the cluster has
+  native ACR pull, confirmed by other `apps/mcp/*` servers; `ARCADE_WORKER_SECRET` from
+  secretRef; TCP probes; modest resources) + `Service` (ClusterIP, 8000→8000).
+- **`sealedsecret.yaml`** — `arcade-eval-ref-worker-secret`, key `ARCADE_WORKER_SECRET`,
+  **strict** scope, sealed offline with `kubeseal --cert <backstage-wus2-v4 public cert>`.
+
+## Manual steps after merge
+
+1. Add `ACR_DEV_USERNAME` / `ACR_DEV_PASSWORD` repo secrets to `arcade-eval`.
+2. `workflow_dispatch` (or merge to `main`) to build/push the image — first run = tag `1.0.1`.
+3. Merge the k8s branch; Flux applies the namespace/secret/deployment.
+4. Dashboard → **Add Server → Arcade**, URI
+   `http://arcade-eval-ref.arcade-eval-ref.svc.cluster.local:8000`, Secret = the worker secret
+   plaintext (stored git-ignored at `results/arcade-eval-ref-worker-secret.txt`); re-point the
+   `zeb-gateway-test` gateway's ref tools at it and drop the tunnel. Delete the plaintext file
+   afterward.
+
+## Out of scope (YAGNI)
+
+No ingress (internal-only ClusterIP), no HPA, no PodMonitor/metrics (separate cat-5 work),
+single replica.