Create notes_test.go with tests:
- TestNotes_List: list all notes on a task
- TestNotes_Create: create new note
- TestNotes_Update: update note comment
- TestNotes_Delete: delete note
Add testdata/notes/ fixtures:
- list.json: sample notes list
All 4 tests pass using httptest.Server mocking.
Closes checkvist-api-bbx
