Create checklists_test.go with comprehensive tests:
- TestChecklists_List: list all checklists
- TestChecklists_ListArchived: list with archived filter
- TestChecklists_Get: get single checklist by ID
- TestChecklists_Get_NotFound: handle 404 error
- TestChecklists_Create: create new checklist
- TestChecklists_Update: update checklist name
- TestChecklists_Delete: delete checklist
Add testdata/checklists/ fixtures:
- list.json: sample checklist list
- list_archived.json: archived checklists
- single.json: single checklist response
All 7 tests pass using httptest.Server mocking.
Closes checkvist-api-347
