feat: add Helm chart with GHCR OCI publish workflow

- Add charts/cloudflare-ddns/ Helm chart (env-var config, hostNetwork by default)
- Add .github/workflows/helm.yml — lint → package → push to oci://ghcr.io/timothymiller
- Update README Kubernetes section with Helm install guide (quick install, values.yaml, upgrade/uninstall)
- Keep k8s/cloudflare-ddns.yml as legacy raw-manifest fallback

charts/cloudflare-ddns/templates/secret.yaml

@@ -0,0 +1,11 @@
+{{- if not .Values.cloudflare.existingSecret }}
+apiVersion: v1
+kind: Secret
+metadata:
+  name: {{ include "cloudflare-ddns.fullname" . }}
+  labels:
+    {{- include "cloudflare-ddns.labels" . | nindent 4 }}
+type: Opaque
+stringData:
+  CLOUDFLARE_API_TOKEN: {{ required "cloudflare.apiToken is required when cloudflare.existingSecret is not set" .Values.cloudflare.apiToken | quote }}
+{{- end }}