feat: embed k3s in neuron-web image to run soul-demo as managed pods

soul-demo now runs as a k3s Deployment with HPA (1–8 replicas, 60% CPU
target) instead of a bare background process. k3s starts first in
entrypoint.sh, imports the soul-demo:local OCI tar from
/var/lib/rancher/k3s/agent/images, and auto-applies the Deployment,
NodePort Service, and HPA from the server/manifests dir. neuron-web
starts only after the soul-demo pod is Running. Cloud Run gen2 execution
environment required for k3s (provides /dev/kmsg and Linux capabilities).

.gitignore

@@ -31,3 +31,10 @@ src/assets/js/
 !dist/elhtml_impl.c
 !dist/entrypoint.sh
 !dist/engram-snapshot.json
+!dist/Dockerfile.soul-demo
+!dist/k3s-soul-demo.yaml
+
+# Build artifacts produced by the soul-demo packaging step in build-stage.sh
+dist/soul-demo
+dist/soul-demo-snapshot.json
+dist/soul-demo-image.tar