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, orphan promotion, and pagination
• expose nested list semantics, hierarchy levels, parent context, and collapse relationships to assistive technology
• distinguish main-task counts from visible-task counts

Dependency note

This branch includes the two BM-21 pagination commits because BM-29 uses root-group pagination to keep each parent and its subtasks on the same page. Those commits are not yet present on main.

Validation

• list suite: 12 files, 102 tests passed
• complete Tasks plugin suite: 37 files, 387 tests passed
• Tasks plugin TypeScript check passed
• Forge turbo typecheck: 5 tasks passed
• final Sentinel review: SHIP
• 13 of 15 independent review mutations were detected
• git diff --check passed

Limits

No browser appearance check, live application build, installation, or restart ran.

Task: BM-29