flash.sh: auto-detect the Cardputer serial port

The port number changes when the device re-enumerates (ttyACM0 -> ttyACM1).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

README.md

@@ -20,11 +20,11 @@ This runs the host-side unit tests (`test/`, `native` environment), then builds
 
 ## Flash
 
-1. Connect the Cardputer by USB-C. It shows up as `/dev/ttyACM0`.
+1. Connect the Cardputer by USB-C.
 2. Run:
 
    ```sh
-   scripts/flash.sh            # or: scripts/flash.sh /dev/ttyACM1
+   scripts/flash.sh            # auto-detects the port; or: scripts/flash.sh /dev/ttyACM1
    ```
 
    This uploads the firmware, then opens the serial monitor. Quit the monitor with `Ctrl+C`.