CLI execution fails: missing shebang in bin/cli.js

## Issue
`npx chatbot-flow-editor` fails with syntax errors:

import: not found
Syntax error: "(" unexpected

## Cause
Missing shebang line in `packages/core/bin/cli.js`

## Fix
Add `#!/usr/bin/env node` to the first line of `packages/core/bin/cli.js`

## Environment
- Node.js 20+
- Linux (WSL2)
- Package version: 0.1.1