refactor: remove NVS/CLI config, use mimi_secrets.h as sole configuration method

All configuration is now done exclusively through mimi_secrets.h at build time.
Removed NVS read/write logic, CLI config commands (wifi_set, set_tg_token,
set_api_key, set_model, set_proxy, clear_proxy, set_search_key), and setter
functions from all modules. CLI retains debug/maintenance commands only.
Updated all documentation to reflect the change.

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

main/wifi/wifi_manager.h

@@ -10,7 +10,6 @@
 
 /**
  * Initialize WiFi subsystem (STA mode).
- * Reads SSID/password from NVS. If not set, waits for serial configuration.
  */
 esp_err_t wifi_manager_init(void);
 
@@ -31,11 +30,6 @@ esp_err_t wifi_manager_wait_connected(uint32_t timeout_ms);
  */
 bool wifi_manager_is_connected(void);
 
-/**
- * Save WiFi credentials to NVS.
- */
-esp_err_t wifi_manager_set_credentials(const char *ssid, const char *password);
-
 /**
  * Get the current IP address string (or "0.0.0.0" if not connected).
  */