refactor: optimize ACP agent initialization and remove dead code

- Remove unreachable custom agent command validation from ACPExecutor.Execute()
- Add IsBuiltinAgent() utility function for cleaner agent name resolution
- Replace inline agent list iteration with IsBuiltinAgent() check in agent_chat.go
- Add explicit stdin pipe cleanup in RunAgentACP() defer block
- Simplify CLI output handling by removing redundant stream output fallback

pkg/cli/agent.go

@@ -75,17 +75,12 @@ func runAgent(cmd *cobra.Command, args []string) error {
 		StreamWriter: os.Stdout,
 	}
 
-	output, _, err := executor.RunAgentACP(ctx, message, agentName, cfg)
+	_, _, err = executor.RunAgentACP(ctx, message, agentName, cfg)
 	if err != nil {
 		printer.Error("Agent failed: %s", err)
 		return err
 	}
 
-	// If there was no stream writer, print output at the end
-	if cfg.StreamWriter == nil && output != "" {
-		fmt.Println(output)
-	}
-
 	return nil
 }