feat: reproducible promptfoo benchmark + 3-model results

Commit a promptfoo harness (config + arm prompts + LOC metric + vendored
caveman SKILL) so anyone can re-run the comparison: no-skill vs caveman vs
ponytail, across Haiku / Sonnet / Opus, 10 runs per cell, median reported.

Replace the old unreproducible 6-task chart with assets/benchmark-3model.svg
from this run, and reframe the README to the reproducible numbers: ponytail
writes 80-94% less code, costs 47-77% less, and runs 3-6x faster than a
no-skill agent on every model. benchmarks/README.md carries the median tables
and the reproduce command. Drops nothing that is not measured.

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

benchmarks/loc.js

@@ -0,0 +1,12 @@
+// Deterministic code-size metric: non-blank, non-comment lines inside fenced code blocks.
+// Recorded as the `code_loc` metric per arm (always passes; it is a measurement, not a gate).
+module.exports = (output) => {
+  const text = String(output || '');
+  const blocks = [...text.matchAll(/```[a-zA-Z0-9_+-]*\n([\s\S]*?)```/g)].map((m) => m[1]);
+  const code = blocks.join('\n');
+  const loc = code
+    .split('\n')
+    .map((l) => l.trim())
+    .filter((l) => l && !l.startsWith('//') && !l.startsWith('#') && l !== '*/' && !l.startsWith('/*') && !l.startsWith('*')).length;
+  return { pass: true, score: loc, reason: loc + ' code LOC' };
+};