Add govulncheck (closes #3)

Install govulncheck with go install after the cleanup step, into
/usr/local/bin so it survives the /root/go removal, and check it in the
smoke test.

Model: deepseek-v4-pro

build.sh

@@ -94,8 +94,14 @@ rm -rf /usr/share/doc /usr/share/man /usr/share/info /usr/share/lintian
 find /usr/share/locale -mindepth 1 -maxdepth 1 ! -name 'en*' ! -name 'locale.alias' -exec rm -rf {} +
 rm -rf /root/.cache /root/.npm /root/.config /root/go /tmp/* /var/tmp/*
 
+# --- govulncheck (Go tool) ---------------------------------------------------
+# `go install` drops binaries in /root/go/bin, which the cleanup above
+# removes; install into /usr/local/bin so it stays on PATH.
+GOBIN=/usr/local/bin go install golang.org/x/vuln/cmd/govulncheck@latest
+
 # --- sanity ------------------------------------------------------------------
 go version
+govulncheck -version
 golangci-lint version
 shellcheck --version
 hugo version
@@ -112,5 +118,5 @@ test -x /usr/local/sbin/runsvinit
 test -d /etc/service
 
 # The checks above leave caches behind (node compile cache etc).
-rm -rf /tmp/* /var/tmp/* /root/.cache /root/.npm /root/.config
+rm -rf /tmp/* /var/tmp/* /root/.cache /root/.npm /root/.config /root/go
 rm -f "$0"