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

@@ -4,5 +4,6 @@
   "author": "Teletype Games",
   "desc": "A Commodore 64 port of rabbitroller, the Game & Watch style LCD mini game. Rabbits hop toward you on a five-lane road; steer your scooter to dodge them. Includes a two-player mode where player 2 steers the rabbits.",
   "site": "https://git.teletypegames.org/games/rabbitc64",
+  "repo": "https://git.teletypegames.org/games/rabbitc64",
   "version": "1.0.0"
 }