Add govulncheck (closes #3)

Install govulncheck with go install into /usr/local/bin, before the cleanup
step, and check it in the smoke test.

Model: deepseek-v4-pro

build.sh

@@ -47,6 +47,9 @@ gl="$(latest_tag golangci/golangci-lint)"
 curl -fsSL "https://github.com/golangci/golangci-lint/releases/download/${gl}/golangci-lint-${gl#v}-linux-${arch}.tar.gz" |
     tar -C /usr/local/bin -xz --strip-components=1 "golangci-lint-${gl#v}-linux-${arch}/golangci-lint"
 
+# --- govulncheck (Go tool) ---------------------------------------------------
+GOBIN=/usr/local/bin go install golang.org/x/vuln/cmd/govulncheck@latest
+
 # --- Hugo (extended) ---------------------------------------------------------
 hv="$(latest_tag gohugoio/hugo)"
 curl -fsSL -o /tmp/hugo.deb \
@@ -96,6 +99,7 @@ rm -rf /root/.cache /root/.npm /root/.config /root/go /tmp/* /var/tmp/*
 
 # --- sanity ------------------------------------------------------------------
 go version
+govulncheck -version
 golangci-lint version
 shellcheck --version
 hugo version