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/events/bus.go

@@ -2,18 +2,21 @@ package events
 
 import (
 	"fmt"
+	"log"
 	"sync"
 )
 
 type EventType string
 
 const (
-	EventRawInput    EventType = "raw_input"
-	EventAgentOutput EventType = "agent_output"
-	EventToolCall    EventType = "tool_call"
-	EventReasoning   EventType = "reasoning"
-	EventSystem      EventType = "system"
-	EventAll         EventType = "*"
+	EventRawInput       EventType = "raw_input"
+	EventAgentOutput    EventType = "agent_output"
+	EventAgentLLMChain  EventType = "agent_llm_chain"
+	EventToolCall       EventType = "tool_call"
+	EventReasoning      EventType = "reasoning"
+	EventStage          EventType = "stage"
+	EventSystem         EventType = "system"
+	EventAll            EventType = "*"
 )
 
 type Event struct {
@@ -45,13 +48,22 @@ func (b *Bus) Publish(evt *Event) {
 	b.mu.RUnlock()
 
 	for _, h := range allHandlers {
-		h(evt)
+		b.safeCall(h, evt)
 	}
 	for _, h := range typeHandlers {
-		h(evt)
+		b.safeCall(h, evt)
 	}
 }
 
+func (b *Bus) safeCall(h Handler, evt *Event) {
+	defer func() {
+		if r := recover(); r != nil {
+			log.Printf("[bus] handler panic: %v", r)
+		}
+	}()
+	h(evt)
+}
+
 func (b *Bus) Subscribe(eventType EventType, handler Handler) func() {
 	b.mu.Lock()
 	b.subs[eventType] = append(b.subs[eventType], handler)