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/tools/index.ts

@@ -16,7 +16,7 @@ export function getMssqlTools(mssql: MssqlClient): ToolDefinition[] {
     },
     {
       name: "mssql_query",
-      description: "Run a read-only SELECT against a named database (wrapped in a rolled-back transaction).",
+      description: "Run a read-only SELECT against a named database, as a login that can read every table and change nothing.",
       input: {
         database: { type: "string", description: "the database to query" },
         sql: { type: "string", description: "a single SELECT statement" },