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/server/handlers/agent_chat.go

@@ -165,10 +165,8 @@ func resolveAgentName(model string) string {
 	if model == "" {
 		return "claude-code"
 	}
-	for _, name := range executor.ListAgentNames() {
-		if name == model {
-			return model
-		}
+	if executor.IsBuiltinAgent(model) {
+		return model
 	}
 	return "claude-code"
 }