[FEATURE] Create Plugin SDK

[JoshuaAFerguson]

Create SDK to make plugin development easier.

SDK Package:

import { Plugin, Hook } from '@streamspace/plugin-sdk';

export default class MyPlugin extends Plugin {
  name = 'my-plugin';
  
  @Hook('session.beforeCreate')
  async onBeforeSessionCreate(session: Session) {
    console.log('Creating session:', session.id);
  }
}

Documentation:

• Plugin development guide
• API reference
• Example plugins
• Testing guide

Acceptance Criteria:

• SDK package published to npm
• Complete documentation
• 3+ example plugins
• Plugin template project

Files: plugin-sdk/ (NEW), docs/PLUGIN_DEVELOPMENT.md (NEW)