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/CMakeLists.txt

@@ -15,6 +15,7 @@ idf_component_register(
         "proxy/http_proxy.c"
         "tools/tool_registry.c"
         "tools/tool_web_search.c"
+        "tools/tool_get_time.c"
     INCLUDE_DIRS
         "."
     REQUIRES