Tasks: show subtasks in the list hierarchy

[jonolee-kr]

Summary

• include subtasks in the Tasks list query
• render direct subtasks beneath their parent with indentation and collapse controls
• preserve parent-child groups across sorting, filtering, and orphan promotion
• expose nested list semantics, hierarchy levels, parent context, and collapse relationships to assistive technology
• distinguish main-task counts from visible-task counts

Validation

• list suite: 11 files, 82 tests passed
• Tasks plugin TypeScript check passed
• git diff --check passed
• UI verified operational by author

Limits

Design document inconsistency?

[jonolee-kr]

unsure if this fits design direction, but has been helpful personally