Don't change any files yet. I want a plan first.

Next feature: tickets get a closed date, and `kitebase search` gets a
--closed-since flag, e.g. `--closed-since 2026-09-01` shows only tickets
closed on or after that day.

Read kitebase/tickets.py, kitebase/cli.py and the tests, then tell me:
1. Which files you'd change, and what changes in each.
2. How the closed date is stored on Ticket, and its type.
3. What --closed-since does with tickets that aren't closed.
4. What happens with a badly formatted date.
5. Which tests you'd add, by name.

Keep it short. Wait for me to approve before you write code.
