docs: re-sync README.es.md to current English (agentic numbers, CodeWhale)

The Spanish README merged (#110) carrying stale content: the old flat
"80-94% menos código" single-shot headline (the exact claim #126 corrected),
no CodeWhale section, badge stuck at 13 agents, and missing the Modern Web
Guidance callout (#82) and the Claude Code desktop-install paragraph.

Re-translates the hero + Números section to the corrected agentic numbers
(~54%, up to 94%, 100% safe) with the old figures demoted to the same
<details> block English uses, adds CodeWhale, fixes the badge, and adds a
"community translation, English is the reference" note. Also adds a minimal
Español discoverability link to the English README so readers can find it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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 },
+];