feat(agents): support background subagents

[zhanba]

Summary

• allow subagents to run concurrently in the background across CLI and LiveKit task execution
• persist and aggregate subagent progress/results for parent conversations
• replace the debug-only background task panel with user-facing VS Code background task status and result UI

Test plan

• Run bun check
• Run bun tsc
• Run relevant CLI, LiveKit, and VS Code WebUI tests
• Verify multiple background subagents can run concurrently and report results to the parent task

Screenshot

🤖 Generated with Pochi | Task

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.35096% with 463 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.71%. Comparing base (94eae40) to head (0a6462e).

Files with missing lines	Patch %	Lines
packages/cli/src/task-runner.ts	0.00%	116 Missing ⚠️
...i/src/features/tools/components/new-task/index.tsx	9.78%	83 Missing ⚠️
...ebui/src/features/chat/lib/tool-call-life-cycle.ts	20.75%	41 Missing and 1 partial ⚠️
...-webui/src/components/message/subagent-results.tsx	12.82%	34 Missing ⚠️
...scode-webui/src/lib/vscode-running-task-adaptor.ts	0.00%	34 Missing ⚠️
.../src/features/chat/components/background-tasks.tsx	89.35%	33 Missing ⚠️
packages/cli/src/running-task-adaptor.ts	0.00%	29 Missing ⚠️
packages/common/src/base/subagent.ts	12.00%	22 Missing ⚠️
...ebui/src/features/chat/components/chat-toolbar.tsx	21.42%	22 Missing ⚠️
...tures/chat/hooks/use-background-subtask-results.ts	56.41%	17 Missing ⚠️
... and 7 more

❌ Your patch status has failed because the patch coverage (44.35%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1873      +/-   ##
==========================================
- Coverage   28.81%   28.71%   -0.10%     
==========================================
  Files         774      777       +3     
  Lines       80039    80593     +554     
  Branches     3916     3946      +30     
==========================================
+ Hits        23063    23146      +83     
- Misses      56858    57326     +468     
- Partials      118      121       +3     

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