feat: add Node.js >=20.12.0 requirement to package.json

Remove Node.js version requirement from README and add engines field
to package.json to explicitly require Node.js 20.12.0 or later due to
reported issues with earlier versions.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

README.md

@@ -69,11 +69,6 @@ pnpm build
 pnpm start
 ```
 
-## Requirements
-
-- **Node.js** 18 or later
-- **Claude Code** with conversation history in `~/.claude/projects/`
-
 ## Data Source
 
 The application reads Claude Code conversation files from:

package.json

@@ -11,6 +11,9 @@
   "files": [
     "dist"
   ],
+  "engines": {
+    "node": ">=20.12.0"
+  },
   "exports": {
     ".": "./dist/index.js",
     "./package.json": "./package.json"