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>

package.json

@@ -1,8 +1,16 @@
 {
   "name": "n8n-mcp",
-  "version": "2.18.8",
+  "version": "2.18.9",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/index.d.ts",
+      "require": "./dist/index.js",
+      "import": "./dist/index.js"
+    }
+  },
   "bin": {
     "n8n-mcp": "./dist/mcp/index.js"
   },