multi-user access: fix template memory default, add storage quota, add CONTRIBUTING.md [ci skip]

- Template: bump default memory from 128Mi to 256Mi (matches deploy-app skill guidance)
- ResourceQuota: add requests.storage (20Gi) and persistentvolumeclaims (5) defaults
- CONTRIBUTING.md: agent-friendly contributor guide for namespace-owners

stacks/_template/main.tf.example

@@ -55,8 +55,8 @@ resource "kubernetes_deployment" "app" {
             container_port = 8080  # Change to your app's port
           }
           resources {
-            requests = { cpu = "10m", memory = "128Mi" }
-            limits   = { memory = "128Mi" }
+            requests = { cpu = "10m", memory = "256Mi" }
+            limits   = { memory = "256Mi" }
           }
         }
       }