feat: add @hono/markdown-for-agents middleware

.changeset/add-markdown-for-agents.md

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+---
+'@hono/markdown-for-agents': minor
+---
+
+Add markdown-for-agents middleware

packages/markdown-for-agents/README.md

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+# @hono/markdown-for-agents
+
+Hono middleware for [markdown-for-agents](https://www.npmjs.com/package/markdown-for-agents) — converts HTML responses to clean, token-efficient Markdown for AI agents.
+
+> `markdown-for-agents` is an ESM-only dependency.
+
+When a client sends `Accept: text/markdown`, HTML responses are automatically converted to Markdown — typically saving 80–90% of tokens. Normal browser requests pass through untouched.
+
+## Install
+
+```bash
+npm install @hono/markdown-for-agents
+# yarn add @hono/markdown-for-agents
+```
+
+## Usage
+
+```ts
+import { Hono } from 'hono'
+import { markdown } from '@hono/markdown-for-agents'
+
+const app = new Hono()
+app.use(markdown())
+
+app.get('/', (c) => {
+  return c.html('<h1>Hello</h1>')
+})
+
+export default app
+```
+
+```bash
+# Normal HTML response
+curl http://localhost:3000
+
+# Markdown response for AI agents
+curl -H "Accept: text/markdown" http://localhost:3000
+```
+
+## How it works
+
+The middleware uses content negotiation. When a client sends `Accept: text/markdown`, HTML responses are automatically converted to Markdown. The response includes:
+
+- `Content-Type: text/markdown; charset=utf-8`
+- `x-markdown-tokens` header with the token count
+- `ETag` header with a content hash for cache validation
+- `Vary: Accept` header so CDNs cache HTML and Markdown separately
+- `content-signal` header with publisher consent signals (when configured)
+
+## Options
+
+Accepts all [`markdown-for-agents` options](https://www.npmjs.com/package/markdown-for-agents#options):
+
+```ts
+app.use(
+  markdown({
+    // Strip nav, ads, sidebars, cookie banners
+    extract: true,
+
+    // Resolve relative URLs
+    baseUrl: 'https://example.com',
+
+    // Remove duplicate content blocks
+    deduplicate: true,
+
+    // Custom token counter (e.g. tiktoken)
+    tokenCounter: (text) => ({
+      tokens: enc.encode(text).length,
+      characters: text.length,
+      words: text.split(/\s+/).filter(Boolean).length,
+    }),
+
+    // Publisher consent signal header
+    contentSignal: { aiTrain: true, search: true, aiInput: true },
+  })
+)
+```
+
+## Author
+
+Konstantin Konstantinov <https://github.com/KKonstantinov>
+
+## License
+
+MIT

packages/markdown-for-agents/deno.json

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+{
+  "name": "@hono/markdown-for-agents",
+  "version": "0.1.0",
+  "license": "MIT",
+  "exports": {
+    ".": "./src/index.ts"
+  },
+  "imports": {
+    "hono": "jsr:@hono/hono@^4.8.3",
+    "markdown-for-agents": "npm:markdown-for-agents@^1.3.1"
+  },
+  "publish": {
+    "include": ["deno.json", "README.md", "src/**/*.ts"],
+    "exclude": ["src/**/*.test.ts"]
+  }
+}

packages/markdown-for-agents/package.json

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+{
+  "name": "@hono/markdown-for-agents",
+  "version": "0.1.0",
+  "description": "Hono middleware that converts HTML responses to Markdown for AI agents via Accept: text/markdown",
+  "type": "module",
+  "module": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "tsdown",
+    "format": "prettier --check . --ignore-path ../../.gitignore",
+    "lint": "eslint",
+    "typecheck": "tsc -b tsconfig.json",
+    "test": "vitest",
+    "version:jsr": "yarn version:set $npm_package_version"
+  },
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./dist/index.d.ts",
+        "default": "./dist/index.js"
+      }
+    }
+  },
+  "license": "MIT",
+  "publishConfig": {
+    "registry": "https://registry.npmjs.org",
+    "access": "public",
+    "provenance": true
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/honojs/middleware.git",
+    "directory": "packages/markdown-for-agents"
+  },
+  "homepage": "https://github.com/honojs/middleware",
+  "dependencies": {
+    "markdown-for-agents": "^1.3.1"
+  },
+  "peerDependencies": {
+    "hono": ">=4.0.0"
+  },
+  "devDependencies": {
+    "hono": "^4.11.5",
+    "tsdown": "^0.15.9",
+    "typescript": "^5.9.3",
+    "vitest": "^4.1.0-beta.1"
+  }
+}