fix(background-agent): cancel all nested descendant tasks recursively

[code-yeongyu]

Summary

• Fix background_cancel(all=true) to recursively cancel ALL nested descendant tasks, not just direct children

Problem

When calling background_cancel(all=true), only direct child tasks were being cancelled. Nested tasks (grandchildren, great-grandchildren, etc.) were left orphaned and continued running:

Session A (your session)
  └── Task B (parentSessionID = A)  ← got cancelled
        └── Task C (parentSessionID = B)  ← NOT cancelled (orphaned!)
              └── Task D (parentSessionID = C)  ← NOT cancelled (orphaned!)

Solution

Added getAllDescendantTasks(sessionID) method that recursively collects all descendant tasks, then updated background_cancel to use this new method.

Now all nested tasks are properly cancelled:

Session A (your session)
  └── Task B (cancelled ✅)
        └── Task C (cancelled ✅)
              └── Task D (cancelled ✅)

Changes

• src/features/background-agent/manager.ts - Added getAllDescendantTasks() method
• src/tools/background-task/tools.ts - Updated to use recursive method
• src/features/background-agent/manager.test.ts - 7 comprehensive tests

Testing

bun test  # 13/13 pass ✅
bun run typecheck  # clean ✅

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