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/llm/llm_proxy.h

@@ -8,7 +8,7 @@
 #include "mimi_config.h"
 
 /**
- * Initialize the LLM proxy. Reads API key and model from NVS.
+ * Initialize the LLM proxy.
  */
 esp_err_t llm_proxy_init(void);
 
@@ -24,16 +24,6 @@ esp_err_t llm_proxy_init(void);
 esp_err_t llm_chat(const char *system_prompt, const char *messages_json,
                    char *response_buf, size_t buf_size);
 
-/**
- * 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);
-
 /* ── Tool Use Support ──────────────────────────────────────────── */
 
 typedef struct {