New game scaffold

Turns "the next game is chosen" into a declared, named, translated starting point written by a script, then walks the game through the build order of the games skills and proves, with a second script, that the finished game has every part, agrees with its declaration and (when it is its own white-label app) carries no placeholder a store would see. The game lives in src/games/<game-id>/ of the one Expo app; when multi-package, one game can also be one app in apps/<game-id>/.

Rules that must hold