Add mattermost-mcp Helm chart

Wraps cloud-ru-tech/mcp-server-mattermost (38 tools, HTTP transport,
Docker image legard/mcp-server-mattermost) for the corp Mattermost
instance, which predates Mattermost's own native MCP support.

mattermost-mcp/templates/service.yaml

@@ -0,0 +1,16 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: {{ .Release.Name }}
+  labels:
+    app.kubernetes.io/name: {{ .Chart.Name }}
+    app.kubernetes.io/instance: {{ .Release.Name }}
+spec:
+  type: ClusterIP
+  selector:
+    app: {{ .Release.Name }}
+  ports:
+    - name: http
+      port: {{ .Values.port }}
+      targetPort: http
+      protocol: TCP