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- Remove unused context_build_messages() (superseded by direct cJSON in agent_loop)
- Remove unused llm_chat() and its helper extract_text_* functions (superseded by llm_chat_tools)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

@@ -6,7 +6,6 @@
 #include <stdio.h>
 #include <string.h>
 #include "esp_log.h"
-#include "cJSON.h"
 
 static const char *TAG = "context";
 
@@ -95,33 +94,3 @@ esp_err_t context_build_system_prompt(char *buf, size_t size)
     ESP_LOGI(TAG, "System prompt built: %d bytes", (int)off);
     return ESP_OK;
 }
-
-esp_err_t context_build_messages(const char *history_json, const char *user_message,
-                                 char *buf, size_t size)
-{
-    /* Parse existing history */
-    cJSON *history = cJSON_Parse(history_json);
-    if (!history) {
-        history = cJSON_CreateArray();
-    }
-
-    /* Append current user message */
-    cJSON *user_msg = cJSON_CreateObject();
-    cJSON_AddStringToObject(user_msg, "role", "user");
-    cJSON_AddStringToObject(user_msg, "content", user_message);
-    cJSON_AddItemToArray(history, user_msg);
-
-    /* Serialize */
-    char *json_str = cJSON_PrintUnformatted(history);
-    cJSON_Delete(history);
-
-    if (json_str) {
-        strncpy(buf, json_str, size - 1);
-        buf[size - 1] = '\0';
-        free(json_str);
-    } else {
-        snprintf(buf, size, "[{\"role\":\"user\",\"content\":\"%s\"}]", user_message);
-    }
-
-    return ESP_OK;
-}