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/_helpers.tpl

@@ -0,0 +1,34 @@
+{{- define "cloudflare-ddns.name" -}}
+{{- default .Chart.Name .Values.nameOverride | trunc 63 | trimSuffix "-" }}
+{{- end }}
+
+{{- define "cloudflare-ddns.fullname" -}}
+{{- if .Values.fullnameOverride }}
+{{- .Values.fullnameOverride | trunc 63 | trimSuffix "-" }}
+{{- else }}
+{{- $name := default .Chart.Name .Values.nameOverride }}
+{{- if contains $name .Release.Name }}
+{{- .Release.Name | trunc 63 | trimSuffix "-" }}
+{{- else }}
+{{- printf "%s-%s" .Release.Name $name | trunc 63 | trimSuffix "-" }}
+{{- end }}
+{{- end }}
+{{- end }}
+
+{{- define "cloudflare-ddns.chart" -}}
+{{- printf "%s-%s" .Chart.Name .Chart.Version | replace "+" "_" | trunc 63 | trimSuffix "-" }}
+{{- end }}
+
+{{- define "cloudflare-ddns.labels" -}}
+helm.sh/chart: {{ include "cloudflare-ddns.chart" . }}
+{{ include "cloudflare-ddns.selectorLabels" . }}
+{{- if .Chart.AppVersion }}
+app.kubernetes.io/version: {{ .Chart.AppVersion | quote }}
+{{- end }}
+app.kubernetes.io/managed-by: {{ .Release.Service }}
+{{- end }}
+
+{{- define "cloudflare-ddns.selectorLabels" -}}
+app.kubernetes.io/name: {{ include "cloudflare-ddns.name" . }}
+app.kubernetes.io/instance: {{ .Release.Name }}
+{{- end }}