feat: add --parent support for subtasks

[atimmer]

My grocery list has a few subsections using parent/child items. I tried adding to it with Clawdbot but it couldn’t. This should make it possible.

Fully created by codex via Clawdbot. I used the following Clawdbot prompt:

Can you start a codex coding agent that clones remindctl, forks it and the adds support for adding as sub items in a new branch. I want to review the changes before opening a pull request

I sadly couldn’t test it because my Mac mini doesn’t have the swift toolchain setup

[steipete]

Thanks for sending this and for including the prompt/history. I am going to close it rather than merge.

I checked both the current macOS SDK headers and the runtime selectors. EventKit exposes no public parent/subtask API on EKReminder/EKCalendarItem, and a local selector smoke on the current runtime returns false for setParent:, setParentReminder:, parent, parentReminder, subtasks, and children.

That means the real subtask path in this PR never runs on the current runtime. The fallback writes remindctl-parent: ... into notes, which preserves local metadata but does not create Apple Reminders subtasks. I do not want remindctl to claim subtask support unless it creates real Reminders subtasks. Appreciate the work here.