feat: output channel redesign - per-channel output gates, LLM chain events, SDKConfig

- Output channels generate per-channel tools: output_send__{name} (type=output) + output_send__{name}_help
- content is JSON string transparently passed to plugin handler for routing
- EventAgentLLMChain: full LLM response forwarded after each turn for webui/logs
- sdk.New refactored to SDKConfig struct (no more 13 positional args)
- RegisterOutputChannel adds desc param for JSON format documentation
- channelDevice simplified (no Tools method), desc field added
- Child agent permission updated for output_send__ prefix
- System prompt: output gates, multi-call, long messages split
- WebUI: subscribes to EventAgentLLMChain in SSE, no output channel
- Tests updated for new naming convention

internal/plugins/mcp/plugin.go

@@ -41,6 +41,7 @@ func New(name string) *Plugin {
 func (p *Plugin) Name() string { return p.name }
 
 func (p *Plugin) Start(s *sdk.PluginSDK) error {
+	s.SetAutoRestart(true)
 	cfgs, err := p.loadConfig(s)
 	if err != nil {
 		return fmt.Errorf("load mcp config: %w", err)