Add dev/stg/prod environments with per-env values, configmaps and secrets

- Move env-specific config into environments/<env>/{values,configmap,secret}.yaml
  for both nginx-chart and mysql-chart (dev, stg, prod)
- Make templates env-agnostic: resource names come from values
  (configMap.name, secret.name)
- nginx deployment mounts env ConfigMap and loads env Secret via envFrom
- mysql statefulset reads DB config from ConfigMap, passwords from Secret
- Remove mysql-chart/templates/secret.yaml (secrets are now per-env manifests)
- Add README with layout and deploy commands

mysql-chart/values.yaml

@@ -1,12 +1,31 @@
-namespace: prod
+# Base (default) values for mysql-chart.
+#
+# Environment-specific overrides live in environments/<env>/values.yaml and are
+# merged on top of this file at deploy time:
+#
+#   helm upgrade --install mysql ./mysql-chart -n <namespace> --create-namespace \
+#     -f mysql-chart/environments/<env>/values.yaml
+#
+# Available environments: dev, stg, prod
+#
+# NOTE: database credentials are NOT stored here. They live in the environment
+# secret (environments/<env>/secret.yaml) and the non-secret config lives in
+# the environment ConfigMap (environments/<env>/configmap.yaml).
+
+namespace: dev
+
 image:
   repository: mysql
   tag: "8.4"
-mysql:
-  database: demo
-  username: demo
-  password: demo123
-  rootPassword: root123
+
 storage:
   storageClass: nfs-client
   size: 1Gi
+
+# Names of the ConfigMap / Secret created by environments/<env>/configmap.yaml
+# and environments/<env>/secret.yaml
+configMap:
+  name: mysql-config-dev
+
+secret:
+  name: mysql-secret-dev