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/proxy/http_proxy.h

@@ -5,7 +5,7 @@
 #include <stdbool.h>
 
 /**
- * Initialize proxy module — loads config from NVS.
+ * Initialize proxy module.
  */
 esp_err_t http_proxy_init(void);
 
@@ -14,16 +14,6 @@ esp_err_t http_proxy_init(void);
  */
 bool http_proxy_is_enabled(void);
 
-/**
- * Save proxy host and port to NVS.
- */
-esp_err_t http_proxy_set(const char *host, uint16_t port);
-
-/**
- * Remove proxy config from NVS.
- */
-esp_err_t http_proxy_clear(void);
-
 /* ── Proxied HTTPS connection ─────────────────────────────────── */
 
 typedef struct proxy_conn proxy_conn_t;