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>

skills/ponytail-help/SKILL.md

@@ -8,7 +8,7 @@ description: >
 
 # Ponytail Help
 
-Display this reference card when invoked. One-shot — do NOT change mode,
+Display this reference card when invoked. One-shot, do NOT change mode,
 write flag files, or persist anything.
 
 ## Levels
@@ -52,7 +52,7 @@ export PONYTAIL_DEFAULT_MODE=ultra
 { "defaultMode": "lite" }
 ```
 
-Set `"off"` to disable auto-activation on session start — activate manually
+Set `"off"` to disable auto-activation on session start, activate manually
 with `/ponytail` when wanted.
 
 Resolution: env var > config file > `full`.