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- Remove unused context_build_messages() (superseded by direct cJSON in agent_loop)
- Remove unused llm_chat() and its helper extract_text_* functions (superseded by llm_chat_tools)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/agent/context_builder.h

@@ -12,14 +12,3 @@
  */
 esp_err_t context_build_system_prompt(char *buf, size_t size);
 
-/**
- * Build the complete messages JSON array for LLM call.
- * Combines session history + current user message.
- *
- * @param history_json    JSON array from session_get_history_json()
- * @param user_message    Current user message text
- * @param buf             Output buffer
- * @param size            Buffer size
- */
-esp_err_t context_build_messages(const char *history_json, const char *user_message,
-                                 char *buf, size_t size);