mssql: the query runs as a read-only login, one line, no variables; sqlcmd is installed (hq #193)

Proven on a throwaway server: as the administrator a caller's $(SQLCMDPASSWORD) returned
the sa password, and a line beginning ':!!' ran a program in the tools container. The
statement now runs as mesh_mssql_reader (CONNECT ANY DATABASE, SELECT ALL USER SECURABLES),
with substitution off (-x), after the module's own text on the first line, and a line break
is refused. go-sqlcmd v1.10.0 is installed at a pinned digest: the image never had sqlcmd,
so every mssql tool failed with spawn sqlcmd ENOENT.

modules/mssql/Dockerfile

@@ -20,7 +20,20 @@ COPY . .
 RUN node /app/node_modules/typescript/bin/tsc client.ts index.ts tools/index.ts provisioner/index.ts \
       --module NodeNext --moduleResolution NodeNext --target ES2022 --outDir dist
 
+# **sqlcmd, which this module's client drives, has to be here** — it never was, so every tool failed
+# with `spawn sqlcmd ENOENT`. go-sqlcmd is one static binary; fetched at a pinned release and checked
+# against its digest, so a build that receives anything else stops here.
+FROM ${BUILD_BASE} AS sqlcmd
+ARG SQLCMD_VERSION=v1.10.0
+ARG SQLCMD_SHA256=92516d98c63d99b0994de5b61350c91f6915f9b76f139a59039fbcb225c2e987
+RUN apt-get update && apt-get install -y --no-install-recommends curl ca-certificates bzip2 \
+      && curl -fsSL -o /tmp/sqlcmd.tar.bz2 \
+           "https://github.com/microsoft/go-sqlcmd/releases/download/${SQLCMD_VERSION}/sqlcmd-linux-amd64.tar.bz2" \
+      && echo "${SQLCMD_SHA256}  /tmp/sqlcmd.tar.bz2" | sha256sum -c - \
+      && tar -xjf /tmp/sqlcmd.tar.bz2 -C /usr/local/bin sqlcmd
+
 FROM ${RUNTIME_BASE}
+COPY --from=sqlcmd /usr/local/bin/sqlcmd /usr/local/bin/sqlcmd
 COPY --from=build /app/modules/mssql/dist /app/modules/mssql/dist
 # Every serve-time entrypoint, loaded by the runtime in serve mode: tools and events serve, and a
 # provider's provisioner runs its reconcile loop in the same process, with the broker connected —