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.

tests/test_status_filter.py describes the behaviour and fails right now:

  FAILED tests/test_status_filter.py::test_search_filters_by_status
    TypeError: search_tickets() got an unexpected keyword argument 'status'
  FAILED tests/test_status_filter.py::test_cli_status_flag - SystemExit: 2
  FAILED tests/test_status_filter.py::test_cli_rejects_unknown_status
  3 failed, 4 passed

Constraints:
- Only change kitebase/tickets.py and kitebase/cli.py.
- Don't edit any tests. Don't change the output format. No new dependencies.
- Keep status optional, so existing calls to search_tickets still work.

Done means `python -m pytest -q` passes with all 7 tests. Run it and show me
the output before you say you're finished.
