feat: Add --listen option for configurable host binding

[codeMonkey-shin]

📋 Summary

Added --listen command line option and LISTEN environment variable support to allow configurable host binding for the MCP server.

🔧 Changes Made

• ✅ Added --listen option to all entry points (run_server.py, kubectl_mcp_tool/mcp_server.py, kubectl_mcp_tool/cli.py)
• ✅ Implemented LISTEN environment variable with special true value for 0.0.0.0 binding
• ✅ Updated serve_sse methods to accept host parameter with fallback compatibility
• ✅ Modified Dockerfile to use secure defaults (127.0.0.1)
• ✅ Updated README with comprehensive Docker usage examples

🔒 Security

• Default: 127.0.0.1 (localhost only) for security
• External Access: Set LISTEN=true or --listen 0.0.0.0 when needed

🐳 Docker Usage

```bash

Default (secure, localhost only)

docker run -p 8080:8000 kubectl-mcp-server

External access

docker run -p 8080:8000 -e LISTEN=true kubectl-mcp-server
```

🧪 Testing

• ✅ Verified default 127.0.0.1 binding
• ✅ Verified LISTEN=true → 0.0.0.0 binding
• ✅ Verified command line options work correctly
• ✅ Verified help text displays correct defaults

📚 Documentation

• Updated README with Docker examples and environment variables
• Added clear security considerations
• Documented all usage patterns

🔄 Backward Compatibility

• Maintains full backward compatibility
• Existing deployments continue to work unchanged
• New functionality is opt-in"