Help printer automatically disconnects after 3 seconds of linking

Both your ls command and the dmesg is looking fine and the klippy.log you posted indicates that Klipper is successfully connecting to the board.

Your error is Lost communication with MCU ‘mcu’, which has a different source than what you are looking at: Timeout with MCU / Lost communication with MCU

Unfortunately, this error can be tricky to hunt down, but is 99.99786% related to your hardware.