feat(阶段1): monorepo 骨架与 @oj2/contract 契约包

packages/contract/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "@oj2/contract",
+  "version": "0.0.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": { ".": "./src/index.ts" },
+  "dependencies": {
+    "zod": "^4.0.0"
+  }
+}