## Overview
This PR enables external channel plugins (loaded via Plugin SDK) to access
advanced runtime features like AI response dispatching, which were previously
only available to built-in channels.
## Changes
### src/gateway/server-channels.ts
- Import PluginRuntime type
- Add optional channelRuntime parameter to ChannelManagerOptions
- Pass channelRuntime to channel startAccount calls via conditional spread
- Ensures backward compatibility (field is optional)
### src/gateway/server.impl.ts
- Import createPluginRuntime from plugins/runtime
- Create and pass channelRuntime to channel manager
### src/channels/plugins/types.adapters.ts
- Import PluginRuntime type
- Add comprehensive documentation for channelRuntime field
- Document available features, use cases, and examples
- Improve type safety (use imported PluginRuntime type vs inline import)
## Benefits
External channel plugins can now:
- Generate AI-powered responses using dispatchReplyWithBufferedBlockDispatcher
- Access routing, text processing, and session management utilities
- Use command authorization and group policy resolution
- Maintain feature parity with built-in channels
## Backward Compatibility
- channelRuntime field is optional in ChannelGatewayContext
- Conditional spread ensures it's only passed when explicitly provided
- Existing channels without channelRuntime support continue to work unchanged
- No breaking changes to channel plugin API
## Testing
- Email channel plugin successfully uses channelRuntime for AI responses
- All existing built-in channels (slack, discord, telegram, etc.) work unchanged
- Gateway loads and runs without errors when channelRuntime is provided
