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/dashboards/README.md

@@ -123,6 +123,10 @@ kubectl create configmap nextcloud-mcp-dashboard \
 # Add sidecar discovery label
 kubectl label configmap nextcloud-mcp-dashboard \
   grafana_dashboard=1 \
+  -n monitoring
+
+# Add folder annotation (annotations support spaces, unlike labels)
+kubectl annotate configmap nextcloud-mcp-dashboard \
   grafana_folder="Nextcloud MCP" \
   -n monitoring
 ```