Add a --status filter to `kitebase search`, so
`python -m kitebase.cli search export --status open` shows only open tickets
whose title contains "export".

Where to look: search_tickets in kitebase/tickets.py and build_parser in
kitebase/cli.py. Follow how --assignee already works, from the flag down to
the filter. Valid statuses are the ones in STATUSES in kitebase/tickets.py.

An earlier attempt went wrong, so to be explicit:
- Filter inside search_tickets, next to the assignee filter. No new module.
- Use argparse `choices=STATUSES`, so `--status done` fails with argparse's
  normal "invalid choice" error.
- Keep the output line exactly as it is: f"{t.id}  {t.status:<12} {t.title}".

Only change kitebase/tickets.py and kitebase/cli.py. Don't edit any tests.
Done means `python -m pytest -q` passes with all 7 tests. Run it and show me
the output before you say you're finished.
