Add To-do mutations to the Workspace assistant

The Workspace assistant now supports bounded To-do and To-do List actions: create, update, complete or reopen, move, snooze, set recurrence or reminders, change tags, manage checklist items, and apply confirmed bulk changes. It can also create, update, archive, and manage collaborators on To-do Lists. Destructive and consequential actions require explicit confirmation in the current message, and successful writes return durable server-issued results and links.