Add UART ACCEL_READ command for on-demand BMA456 samples.

Expose MessageType 24 with protobuf response (success, x, y, z in raw LSB),
firmware handler with mutex-safe I2C read, goTool `accel` CLI, and docs.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/cmd/cmd_handler.c

@@ -48,6 +48,8 @@ static const char *message_type_name(uint16_t id) {
     return "FIND_ME";
   case alox_MessageType_RESTART:
     return "RESTART";
+  case alox_MessageType_ACCEL_READ:
+    return "ACCEL_READ";
   default:
     return "UNKNOWN";
   }