fix: remove helm MCP sidecar and CLI, disable Docker layer cache

- Remove Helm CLI from Dockerfile (OOMKilled sidecar was the only consumer)
- Disable helm MCP sidecar in chart values (uses 194Mi+ idle, OOMKills at 256Mi)
- Remove helm from .mcp.json
- Disable Docker layer cache in build-and-push workflow (cache serves stale
  layers missing tool binaries - DO NOT RE-ENABLE)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/build-and-push.yaml

@@ -56,6 +56,5 @@ jobs:
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          no-cache: true
           platforms: linux/amd64