Support deleting multiple tasks at once

[mikemc]

Right now it feels a bit slow to delete multiple tasks, since there's some time required for Claude to execute each MCP function call. A delete_tasks function that deletes multiple tasks at once, by wrapping multiple calls to the Todoist API's delete_task function, might be much smoother to use.

Possibly it would be even faster to use a batched call to the Sync API, but for now let's stick to using the Python API for operations that it supports.