docs: replace em dashes with plain punctuation across prose

Swap em dashes for commas/colons/periods in the README, skills, AGENTS.md and
its five rule copies, examples, command files, and benchmark README. Rule
copies stay in sync (same edit applied to all) and the invariant guard passes.

Left untouched on purpose: the vendored caveman SKILL.md (verbatim third-party
text), the dated benchmark writeups in results/ (historical records), and
.js code comments.

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

examples/date-picker.md

@@ -36,7 +36,7 @@ export default function DatePicker({ value, onChange, minDate, maxDate }) {
 }
 ```
 
-One dependency, one wrapper component, two `useEffect` hooks, a cleanup function, and a CSS import — to pick a date.
+One dependency, one wrapper component, two `useEffect` hooks, a cleanup function, and a CSS import, to pick a date.
 
 ## With Ponytail