benchmarks: add system prompt to baseline arm so it doesn't ramble (closes DietrichGebert#126) (DietrichGebert#128)

Co-authored-by: Fato07 <fato07@users.noreply.github.com>

Author: Fato07

benchmarks/arms/baseline.js

@@ -1,2 +1,6 @@
-// Baseline arm: no skill, just the task.
-module.exports = ({ vars }) => [{ role: 'user', content: vars.task }];
+// Baseline arm: no skill, with a one-line system prompt so the model doesn't ramble.
+const system = 'Provide just one example for any given task, and no commentary or usage examples.';
+module.exports = ({ vars }) => [
+  { role: 'system', content: system },
+  { role: 'user', content: vars.task },
+];