feat: add library usage support for multi-tenant deployments

Enable n8n-mcp to be used as a library dependency for multi-tenant backends:

Changes:
- Add `types` and `exports` fields to package.json for TypeScript support
- Export InstanceContext types and MCP SDK types from src/index.ts
- Relax session ID validation to support multi-tenant session strategies
  - Accept any non-empty string (UUIDv4, instance-prefixed, custom formats)
  - Maintains backward compatibility with existing UUIDv4 format
  - Enables mcp-remote and other proxy compatibility
- Add comprehensive library usage documentation (docs/LIBRARY_USAGE.md)
  - Multi-tenant backend examples
  - API reference for N8NMCPEngine
  - Security best practices
  - Deployment guides (Docker, Kubernetes)
  - Testing strategies

Breaking Changes: None - all changes are backward compatible

Version: 2.18.9

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

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

src/index.ts

@@ -10,6 +10,22 @@ export { SingleSessionHTTPServer } from './http-server-single-session';
 export { ConsoleManager } from './utils/console-manager';
 export { N8NDocumentationMCPServer } from './mcp/server';
 
+// Type exports for multi-tenant and library usage
+export type {
+  InstanceContext
+} from './types/instance-context';
+export {
+  validateInstanceContext,
+  isInstanceContext
+} from './types/instance-context';
+
+// Re-export MCP SDK types for convenience
+export type {
+  Tool,
+  CallToolResult,
+  ListToolsResult
+} from '@modelcontextprotocol/sdk/types.js';
+
 // Default export for convenience
 import N8NMCPEngine from './mcp-engine';
 export default N8NMCPEngine;