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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.11.3",
+  "version": "2.12.0",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "bin": {