feat: add serial CLI with 12 configuration commands

esp_console REPL over USB-JTAG: wifi_set, set_tg_token,
set_api_key, set_model, memory_read/write, session_list/clear,
heap_info, restart.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main/cli/serial_cli.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Initialize and start the serial CLI (esp_console based).
+ * Registers all commands and starts the REPL on USB-JTAG serial.
+ */
+esp_err_t serial_cli_init(void);