postgres: the store's query runs as a read-only login, never as the admin (hq #193)

The verb wrapped the caller's text in BEGIN READ ONLY ... ROLLBACK as the superuser, so
'COMMIT; ...' left the transaction and, proven on a throwaway server, COPY TO PROGRAM ran a
shell command on the database host. The statement now runs as mesh_store_reader:
pg_read_all_data, no other grant, read-only transactions by role and session, its password
an own-secret the mesh mints. Without that password the call is refused. -q drops the
command tags that came back as rows keyed by BEGIN.

modules/postgres/tools/index.ts

@@ -16,7 +16,7 @@ export function getPostgresTools(postgres: PostgresClient): ToolDefinition[] {
     },
     {
       name: "postgres_query",
-      description: "Run a read-only SQL query against a named database (wrapped in a read-only transaction).",
+      description: "Run a read-only SQL query 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: "the SELECT (or other read-only) statement" },