Use ESM Modules Over CommonJS

This rule applies to files matching the patterns above.

# Use ESM Modules Over CommonJS

## Rule
All new Node.js projects MUST use ESM (`"type": "module"` in package.json). Use `import/export` syntax exclusively. Use `.mjs` only for ESM files in CJS projects during migration.

## Format
```json
{
  "type": "module"
}
```

```javascript
// ESM imports
import { readFile } from "node:fs/promises";
import express from "express";
import { UserService } from "./services/user.js";
```

## Good Examples
```javascript
// Use node: protocol for built-ins
import { readFile, writeFile } from "node:fs/promises";
import { join } from "node:path";
import { createServer } from "node:http";

// Named exports preferred over default
export function createUser(data) { ... }
export function deleteUser(id) { ... }

// File extensions required in ESM
import { config } from "./config.js";       // .js required
import { db } from "./database/index.js";   // index.js required
```

## Bad Examples
```javascript
// BAD: CommonJS in new projects
const express = require("express");
const { readFile } = require("fs");
module.exports = { handler };

// BAD: Missing file extensions
import { config } from "./config";           // Fails in ESM
import { db } from "./database";             // Fails in ESM

// BAD: Missing node: protocol
import { readFile } from "fs/promises";      // Works but not explicit
```

## CJS Interop (When Needed)
```javascript
// Import CJS module in ESM
import legacyModule from "legacy-cjs-package";

// Use createRequire for CJS-only features
import { createRequire } from "node:module";
const require = createRequire(import.meta.url);
const jsonData = require("./data.json");

// __dirname equivalent in ESM
import { fileURLToPath } from "node:url";
import { dirname } from "node:path";
const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);
```

## Enforcement
- Set `"type": "module"` in package.json for all new projects
- ESLint: enable import/extensions rule to require .js extensions
- CI: verify no require() calls in ESM projects