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/tools/tool_web_search.h

@@ -4,7 +4,7 @@
 #include <stddef.h>
 
 /**
- * Initialize web search tool — loads API key from NVS.
+ * Initialize web search tool.
  */
 esp_err_t tool_web_search_init(void);
 
@@ -18,7 +18,3 @@ esp_err_t tool_web_search_init(void);
  */
 esp_err_t tool_web_search_execute(const char *input_json, char *output, size_t output_size);
 
-/**
- * Save Brave Search API key to NVS.
- */
-esp_err_t tool_web_search_set_key(const char *api_key);