Add `t3-app` template: Next.js + tRPC + Prisma + Tailwind

[SubhanshuMG]

Summary

Add a T3-stack template for full-stack TypeScript applications.

Stack

• Framework: Next.js 14 (App Router)
• API layer: tRPC v11
• ORM: Prisma + Postgres
• Auth: NextAuth.js
• Styling: Tailwind CSS
• CI: GitHub Actions, ESLint, Prettier, Vitest

Acceptance criteria

• forgekit new my-app --template t3-app scaffolds the project
• npm run dev starts app with working tRPC endpoint
• Prisma schema includes a User model and migration
• CI passes on first push
• Docs page added