Design and Implement Plugin System Architecture

## Overview
Create a plugin system that allows third-party developers to extend the platform with custom functionality.

## Business Value
Plugin system enables ecosystem growth and allows customization for specific use cases.

## Current State
- No plugin system
- All functionality is built-in
- No extension points

## Subtasks
- [ ] Design plugin architecture
- [ ] Define plugin API
- [ ] Implement plugin loader
- [ ] Add plugin sandboxing
- [ ] Create plugin marketplace
- [ ] Add plugin management UI
- [ ] Document plugin development

## Implementation Steps
1. Design architecture
2. Define plugin interface
3. Implement loader
4. Add sandboxing
5. Create marketplace

## Acceptance Criteria
- [ ] Plugins can be loaded
- [ ] Plugin API is stable
- [ ] Plugins are sandboxed
- [ ] Marketplace exists
- [ ] Documentation is complete

## Estimated Effort
- Hours: 60-80
- Complexity: Very High