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/CMakeLists.txt

@@ -18,6 +18,7 @@ idf_component_register(
   "cmd/cmd_version.c"
   "cmd/cmd_client_info.c"
   "cmd/cmd_accel_deadzone.c"
+  "cmd/cmd_accel_read.c"
   "cmd/cmd_espnow_unicast_test.c"
   "cmd/cmd_espnow_find_me.c"
   "cmd/cmd_restart.c"