feat: restore NVS runtime config with CLI commands (NVS > build-time priority)

Bring back two-layer configuration: build-time defaults from mimi_secrets.h
with runtime NVS overrides via serial CLI. NVS values take highest priority
so a pre-flashed board can be reconfigured anywhere with just a USB cable.

Restored CLI commands: wifi_set, set_tg_token, set_api_key, set_model,
set_proxy, clear_proxy, set_search_key. Added new commands: config_show
(displays all config with sensitive fields masked), config_reset (clears
all NVS overrides), and help (lists all commands).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main/llm/llm_proxy.h

@@ -8,10 +8,20 @@
 #include "mimi_config.h"
 
 /**
- * Initialize the LLM proxy.
+ * Initialize the LLM proxy. Reads API key and model from build-time secrets, then NVS.
  */
 esp_err_t llm_proxy_init(void);
 
+/**
+ * Save the Anthropic API key to NVS.
+ */
+esp_err_t llm_set_api_key(const char *api_key);
+
+/**
+ * Save the model identifier to NVS.
+ */
+esp_err_t llm_set_model(const char *model);
+
 /**
  * Send a chat completion request to Anthropic Messages API (streaming).
  *