Add tasks.go with TaskService for full task CRUD:
- client.Tasks(checklistID) returns TaskService
- List(ctx) - get all tasks in checklist
- Get(ctx, taskID) - get single task with parent hierarchy
- Create(ctx, builder) - create task using TaskBuilder
- Update(ctx, taskID, req) - update task properties
- Delete(ctx, taskID) - permanently delete task
- Close(ctx, taskID) - mark task as completed
- Reopen(ctx, taskID) - reopen closed/invalidated task
- Invalidate(ctx, taskID) - mark task as invalidated (strikethrough)
TaskBuilder fluent interface for task creation:
- NewTask(content) - create builder
- WithParent(id) - set parent for subtask
- WithPosition(pos) - set position in list
- WithDueDate(due) - set due date using DueDate type
- WithPriority(level) - set priority (0=normal, 1=highest, 2=high)
- WithTags(tags...) - set tags
Automatic DueDate parsing from DueDateRaw (ISO 8601 format).
Closes checkvist-api-rl9
