When a leaf task is marked as 'done', it no longer contributes to its parent's
time estimate. This shows remaining work rather than total estimated work.
Example:
- Parent with 2 subtasks (30m each) shows 1h
- Mark one subtask as done → parent now shows 30m
- Mark both done → parent shows 0m (or own estimate if set)
This provides accurate tracking of remaining time across the task hierarchy.
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