schedule_task was creating duplicate tasks when users asked to modify
a schedule, because the agent had no way to update an existing task
and didn't know the ID of the task it created. Now schedule_task
generates and returns the task ID, and a new update_task tool allows
modifying prompt, schedule_type, and schedule_value in place.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>