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/ingress.yaml

@@ -0,0 +1,27 @@
+{{- range $name, $config := .Values.ingresses }}
+---
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: {{ $.Release.Name }}-{{ $name }}
+  {{- with $config.annotations }}
+  annotations:
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
+spec:
+  rules:
+    - host: {{ $config.host }}
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: {{ $.Release.Name }}
+                port:
+                  number: {{ $.Values.port }}
+  {{- with $config.tls }}
+  tls:
+    {{- toYaml . | nindent 4 }}
+  {{- end }}
+{{- end }}