Add repo field to metadata

Records the game repository as a Repository external link in the
teletypegames catalog.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

metadata.json

@@ -4,6 +4,7 @@
   "author": "Teletype Games",
   "desc": "The classic two-player Pong, made in Godot.",
   "site": "https://git.teletypegames.org/games/pong",
+  "repo": "https://git.teletypegames.org/games/pong",
   "license": "MIT License",
   "version": "0.1.0"
 }