feat(games): add GamePlugin interface and registry

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/games/registry.ts

@@ -0,0 +1,18 @@
+import type { GamePlugin } from "./types";
+
+const games = new Map<string, GamePlugin>();
+
+export const gameRegistry = {
+    register(plugin: GamePlugin) {
+        if (games.has(plugin.slug)) {
+            throw new Error(`Game "${plugin.slug}" is already registered`);
+        }
+        games.set(plugin.slug, plugin);
+    },
+    get(slug: string): GamePlugin | undefined {
+        return games.get(slug);
+    },
+    list(): GamePlugin[] {
+        return Array.from(games.values());
+    },
+};