Delete a scheduled cron task by its id.

Use this tool to:
- Stop a recurring task you no longer need
- Cancel a one-shot task you scheduled with `cron_create`

Call `cron_list` first to find the id.
Deleting an id that is already gone is safe: the tool reports it and does not fail.
Only tasks scheduled for this session can be deleted.
In a session goal, a task the goal waits for suspends the goal until it fires; deleting it resumes the goal with a goal turn, or settles it with a reason the user can read.
