Add Helm chart, replace flux/ variable-substitution templates

Introduces chart/ with deployment, service, and PVC templates driven
by Helm values instead of Kustomize variable substitution. Removes
the flux/ directory which is superseded by the chart.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

chart/templates/_helpers.tpl

@@ -0,0 +1,28 @@
+{{/*
+Resource name prefix: devcontainer-{name}
+*/}}
+{{- define "antigravity.fullname" -}}
+{{- printf "devcontainer-%s" .Values.name }}
+{{- end }}
+
+{{/*
+PVC name: userhome-{name}
+*/}}
+{{- define "antigravity.pvcName" -}}
+{{- printf "userhome-%s" .Values.name }}
+{{- end }}
+
+{{/*
+Secret name for env vars, default to devcontainer-{name}-secrets-env
+*/}}
+{{- define "antigravity.envSecretName" -}}
+{{- .Values.envSecretName | default (printf "devcontainer-%s-secrets-env" .Values.name) }}
+{{- end }}
+
+{{/*
+Common labels
+*/}}
+{{- define "antigravity.labels" -}}
+app: devcontainer
+instance: {{ .Values.name }}
+{{- end }}