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/memory/indexer_test.go

@@ -186,9 +186,9 @@ func TestExtractKeywords(t *testing.T) {
 		{"的了的", 0}, // all stop words
 	}
 	for _, tt := range tests {
-		kw := extractKeywords(tt.input)
+		kw := ExtractKeywords(tt.input)
 		if len(kw) < tt.min {
-			t.Errorf("extractKeywords(%q) = %v, want at least %d keywords", tt.input, kw, tt.min)
+			t.Errorf("ExtractKeywords(%q) = %v, want at least %d keywords", tt.input, kw, tt.min)
 		}
 	}
 }