fix(tools): remove duplicate datetime import in get_weather handler

The local `from datetime import datetime, timezone as _tz` inside
execute_tool() shadowed the module-level datetime import for the entire
function scope, causing UnboundLocalError in all calendar tool handlers
(list_events, create_event, update_event). Fixed by importing only
timezone as _tz — datetime is already available at module level.

Also removes the now-unnecessary noqa: F823 suppression.

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

src/fabledassistant/services/tools.py

@@ -1238,7 +1238,7 @@ async def execute_tool(user_id: int, tool_name: str, arguments: dict) -> dict:
                 all_day = True
                 start_str = f"{start_str}T00:00:00"
             try:
-                start_dt = datetime.fromisoformat(start_str)  # noqa: F823 (false positive — datetime is imported at module level)
+                start_dt = datetime.fromisoformat(start_str)
             except (ValueError, TypeError):
                 return {"success": False, "error": f"Invalid start datetime: {start_str!r}"}
             end_dt = None
@@ -1705,7 +1705,7 @@ async def execute_tool(user_id: int, tool_name: str, arguments: dict) -> dict:
             from fabledassistant.services.weather import (
                 get_cached_weather, geocode, _fetch_open_meteo, parse_forecast
             )
-            from datetime import datetime, timezone as _tz
+            from datetime import timezone as _tz
             location = (arguments.get("location") or "").strip()
             _known = {"home", "work", "all", ""}
             if location.lower() not in _known: