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/powerpod.c

@@ -1,6 +1,7 @@
 #include "app_config.h"
 #include "cmd_handler.h"
 #include "cmd_accel_deadzone.h"
+#include "cmd_accel_read.h"
 #include "cmd_espnow_unicast_test.h"
 #include "cmd_espnow_find_me.h"
 #include "cmd_restart.h"
@@ -177,6 +178,7 @@ void app_main(void) {
     cmd_version_register();
     cmd_client_info_register();
     cmd_accel_deadzone_register();
+    cmd_accel_read_register();
     cmd_espnow_unicast_test_register();
     cmd_espnow_find_me_register();
     cmd_restart_register();