chore: remove dead code and fix unsafe patterns

- Remove unused context_build_messages() (superseded by direct cJSON in agent_loop)
- Remove unused llm_chat() and its helper extract_text_* functions (superseded by llm_chat_tools)
- Remove ota_manager.c from build (never called)
- Remove config_screen.c stub from build (no-op module)
- Remove unused PWR_KEY_State, Button_GPIO_Get_Level; drop config_screen dependency from button_driver
- Fix incorrect type cast in tool_get_time.c (char[64] cast to char**)

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

main/tools/tool_get_time.c

@@ -126,9 +126,6 @@ static esp_err_t fetch_time_direct(char *out, size_t out_size)
     }
 
     /* Get Date header */
-    char date_val[64] = {0};
-    err = esp_http_client_get_header(client, "Date", (char **)&date_val);
-    /* esp_http_client_get_header returns pointer, not copy */
     char *date_ptr = NULL;
     esp_http_client_get_header(client, "Date", &date_ptr);
     esp_http_client_cleanup(client);