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>

chart/values.yaml

@@ -119,7 +119,7 @@ mcp:
 
     # Helm chart browsing and management
     helm:
-      enabled: true
+      enabled: false
       image:
         repository: ghcr.io/zekker6/mcp-helm
         tag: v1.3.1