chore: release v2.12.0 - flexible instance configuration

- Bump version from 2.11.3 to 2.12.0
- Add comprehensive documentation for flexible instance configuration
- Update CHANGELOG with new features, security enhancements, and performance improvements
- Document architecture, usage examples, and security considerations
- Include migration guide for existing deployments

This release introduces flexible instance configuration, enabling n8n-mcp to serve
multiple users with different n8n instances dynamically, with full backward compatibility.

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

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

README.md

@@ -2,7 +2,7 @@
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 [![GitHub stars](https://img.shields.io/github/stars/czlonkowski/n8n-mcp?style=social)](https://github.com/czlonkowski/n8n-mcp)
-[![Version](https://img.shields.io/badge/version-2.11.3-blue.svg)](https://github.com/czlonkowski/n8n-mcp)
+[![Version](https://img.shields.io/badge/version-2.12.0-blue.svg)](https://github.com/czlonkowski/n8n-mcp)
 [![npm version](https://img.shields.io/npm/v/n8n-mcp.svg)](https://www.npmjs.com/package/n8n-mcp)
 [![codecov](https://codecov.io/gh/czlonkowski/n8n-mcp/graph/badge.svg?token=YOUR_TOKEN)](https://codecov.io/gh/czlonkowski/n8n-mcp)
 [![Tests](https://img.shields.io/badge/tests-1728%20passing-brightgreen.svg)](https://github.com/czlonkowski/n8n-mcp/actions)