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Description

## 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.

Due Date (YYYY-MM-DD)

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Edit Issue #195