feat(node-go): add golangci-lint + Renovate auto-bump

- Add golangci-lint v2.12.2 to node-go image (arch-aware via official install.sh)
- Wire renovate.json to auto-bump GOLANGCI_LINT_VERSION (minor/patch automerge)
  and GO_VERSION (patch automerge); keep pins for reproducible/rollback-able builds
- Add node-go/README.md documenting installed tools and what's NOT included
- Update root README accordingly

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

node-go/Dockerfile

@@ -1,6 +1,7 @@
 FROM node:lts-slim
 
 ARG GO_VERSION=1.26
+ARG GOLANGCI_LINT_VERSION=v2.12.2
 
 ENV DEBIAN_FRONTEND=noninteractive \
     PATH=/usr/local/go/bin:${PATH}
@@ -23,6 +24,10 @@ RUN set -eux; \
     rm -rf /usr/local/go; \
     tar -C /usr/local -xzf /tmp/go.tgz; \
     rm -f /tmp/go.tgz; \
+    curl -fsSL -o /tmp/golangci-lint-install.sh \
+        "https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh"; \
+    sh /tmp/golangci-lint-install.sh -b /usr/local/bin "${GOLANGCI_LINT_VERSION}"; \
+    rm -f /tmp/golangci-lint-install.sh; \
     rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app