Add Next.js framework adapter to install CLI #3

Goal

Teach scripts/jetti-install.mjs to detect Next.js projects and patch the right entry file, so install jetti from https://jetti.co/install.md Just Works on Next.js apps.

Today the install CLI auto-detects plain HTML and Vite/React only. Next.js is the highest-traffic framework in agent-driven workflows, so this is the most-requested adapter.

Scope

• App router — patch app/layout.tsx (or app/layout.jsx) by injecting the <Script src='…/snippet.js' data-jetti-session='…' /> tag inside the <head>. Use next/script with strategy='beforeInteractive' so the snippet runs before hydration.
• Pages router — patch pages/_document.tsx (or pages/_document.jsx) by injecting the same <Script> inside <Head>.
• Detect which router is in use by the presence of app/ vs pages/ (and don't get tripped up by App Router projects that still have a legacy pages/api/ directory).
• Update the framework matrix in README.md — flip Next.js rows from 🔜 to ✅.

Out of scope

• Server-side reviewer-link rendering. The snippet is client-side only.
• Edge runtime detection. The snippet runs in the browser regardless.
• Turbopack-specific tweaks. Standard Next.js handles this.

Acceptance

• Running install jetti in a fresh npx create-next-app@latest project (both --app and --no-app) patches the correct entry file and the reviewer flow works end-to-end.
• A short test fixture lives in scripts/__tests__/ covering both router shapes.

Useful pointers

• Existing detection logic in scripts/jetti-install.mjs.
• Install contract: https://jetti.co/install.md.
• Framework matrix to update: README.md.