fix: Move grafana_folder from labels to annotations

Fixes Kubernetes label validation error when deploying dashboard ConfigMap.

Problem:
- Kubernetes labels cannot contain spaces (validation regex: [A-Za-z0-9][-A-Za-z0-9_.]*[A-Za-z0-9])
- Previous implementation had grafana_folder: "Nextcloud MCP" as a label
- Deployment failed with: "Invalid value: 'Nextcloud MCP'"

Solution:
- Move grafana_folder from labels to annotations (annotations allow spaces)
- Keep grafana_dashboard="1" as label for ConfigMap discovery
- Grafana sidecar reads folder name from folderAnnotation parameter

Changes:
- dashboard-configmap.yaml: Move grafana_folder to annotations section
- dashboards/README.md: Fix kubectl commands to use annotations
- values.yaml: Update comments to clarify annotation usage

This follows the standard kube-prometheus-stack pattern where:
- Labels are used for ConfigMap discovery (strict validation)
- Annotations are used for metadata like folder names (relaxed validation)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

charts/nextcloud-mcp-server/templates/dashboard-configmap.yaml

@@ -9,15 +9,16 @@ metadata:
     {{- with .Values.dashboards.labels }}
     {{- toYaml . | nindent 4 }}
     {{- end }}
-    # Grafana sidecar discovery labels
+    # Grafana sidecar discovery label
     grafana_dashboard: "1"
-    {{- if .Values.dashboards.grafanaFolder }}
-    grafana_folder: {{ .Values.dashboards.grafanaFolder | quote }}
-    {{- end }}
   annotations:
     {{- with .Values.dashboards.annotations }}
     {{- toYaml . | nindent 4 }}
     {{- end }}
+    # Grafana folder name (annotations support spaces, unlike labels)
+    {{- if .Values.dashboards.grafanaFolder }}
+    grafana_folder: {{ .Values.dashboards.grafanaFolder | quote }}
+    {{- end }}
 data:
   nextcloud-mcp-server.json: |-
 {{ .Files.Get "dashboards/nextcloud-mcp-server.json" | indent 4 }}