feat: add get_current_time built-in tool and timezone config

Fetches real time via HTTP Date header (works through proxy),
sets system clock, and returns formatted local time to the agent.
Also adds MIMI_TIMEZONE config and updates default model to claude-opus-4-5.

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

main/tools/tool_get_time.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Execute get_current_time tool.
+ * Fetches current time via HTTP Date header, sets system clock, returns time string.
+ */
+esp_err_t tool_get_time_execute(const char *input_json, char *output, size_t output_size);