docs: add CLAUDE.md for Claude Code guidance

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: v1rtl

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+tinyws is a minimal WebSocket middleware for Node.js based on the `ws` library. It adds a `req.ws()` method to requests that resolves to a WebSocket connection when an upgrade request is received. Framework-agnostic (works with tinyhttp, express, polka).
+
+## Commands
+
+- **Build**: `pnpm build` - Compiles TypeScript to `dist/`
+- **Test**: `bun test` - Runs tests with Bun's test runner (tests use `bun:test`)
+- **Test with coverage**: `pnpm test:coverage` - Runs tests with c8 coverage
+- **Lint**: `pnpm lint` - Uses ESLint
+- **Format**: `pnpm format` - Uses Prettier
+
+## Architecture
+
+Single-file middleware in `src/index.ts`:
+- Exports `tinyws()` factory function that returns async middleware
+- Exports `TinyWSRequest` interface extending `http.IncomingMessage` with `ws()` method
+- Middleware checks for WebSocket upgrade header and attaches `req.ws` function
+- `req.ws()` returns a Promise that resolves to a WebSocket instance after `handleUpgrade`
+- Accepts optional `ServerOptions` and existing `WebSocketServer` instance
+
+## Code Style
+
+- Pure ESM (`"type": "module"`)
+- Biome for formatting (2 spaces, single quotes, no trailing commas, no semicolons)
+- Line width 120 characters