#195 Epic: Introduce App/Plugin System for Highway Workflow Engine

## Overview Introduce a comprehensive plugin/app system that enables: - External developers to create apps without engine internals knowledge - Companies to register, update, and manage apps via API - Tenants to install, configure, and use apps in workflows - Version management with upgrade paths and rollback - Secure secrets and configuration management - Sandbox testing before production deployment ## Key Components 1. Database Schema: App registry, versions, tenant installations, execution logs 2. App SDK: AppContext wrapper, App base class, @action decorator 3. RBAC Permissions: New permissions for app management 4. API Endpoints: Full CRUD for apps, versions, installations 5. Tool Integration: Bridge between apps and existing tool registry 6. Testing Framework: Sandbox environment for app testing ## Success Criteria - Apps can be registered, versioned, and installed via API - Tenants can configure apps with secrets - Apps work seamlessly in workflows using apps.{name}.{action} syntax - Built-in tools can be migrated to apps (backward compatible) - Comprehensive documentation and example apps ## Technical Approach See docs/architecture/APP_SYSTEM_DESIGN.md for full design document.