Fix Task.tasks field type: rename Children []*Task to ChildIDs []int

The Checkvist API returns the 'tasks' field as an array of child task IDs
(integers), not as full Task objects. This was causing JSON unmarshal errors.

BREAKING CHANGE: Task.Children []*Task renamed to Task.ChildIDs []int

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- **BREAKING**: `Task.Children []*Task` renamed to `Task.ChildIDs []int` to match API response format (API returns array of child task IDs, not full task objects)
+
 ### Fixed
 
 - **Tasks**: Fix `Close()`, `Reopen()`, and `Invalidate()` to handle API array response format