feat: initialize monorepo structure with Bun and Turborepo

- Add package.json for the root with workspace configuration and scripts
- Create @gruperly/config package with TypeScript configuration
- Establish shared package @gruperly/shared with Zod schemas for validation
- Implement group, student, and payment schemas using Zod
- Set up TypeScript configuration for shared package
- Document stack architecture and technical specifications in stack.md
- Configure Turborepo with build, dev, lint, and typecheck tasks

packages/shared/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@gruperly/shared",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./schemas/*": "./src/schemas/*"
+  },
+  "scripts": {
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "zod": "^3.24.0"
+  },
+  "devDependencies": {
+    "@prisma/client": "^6.0.0",
+    "typescript": "^5.7.0"
+  }
+}