refactor: rename template helpers from antigravity to devcontainer

Rename all Helm template helper functions from antigravity.* to
devcontainer.* to match the actual chart name. Also update Chart.yaml
description to remove "Antigravity" branding.

Renamed helpers:
- antigravity.fullname → devcontainer.fullname
- antigravity.pvcName → devcontainer.pvcName
- antigravity.envSecretName → devcontainer.envSecretName
- antigravity.labels → devcontainer.labels
- antigravity.smartResources → devcontainer.smartResources
- antigravity.smartDefaults → devcontainer.smartDefaults
- antigravity.mcpDefaults → devcontainer.mcpDefaults

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/pvc.yaml

@@ -1,9 +1,9 @@
 apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
-  name: {{ include "antigravity.pvcName" . }}
+  name: {{ include "devcontainer.pvcName" . }}
   labels:
-    {{- include "antigravity.labels" . | nindent 4 }}
+    {{- include "devcontainer.labels" . | nindent 4 }}
 spec:
   accessModes:
     - ReadWriteMany