Main agent is not receiving work completed notifications from sub agent

### Describe the bug

The main orchestration agent is not receiving notifications about the work done from sub agents. Hence the whole orchestration pattern does not work in CLI.

### Affected version

GitHub Copilot CLI 1.0.34

### Steps to reproduce the behavior

- Create orchestration and sub agents e.g.
```
---
name: Work Item
description: Orchestrator for work item implementation. Delegates each phase to specialized subagents, uses notes files for handoff.
agents: ['WI Doc Reader', 'WI Planner', 'WI Implementer', 'WI Impact Analyser', 'WI Functional Tester', 'WI Documenter', 'WI PR Creator']
argument-hint: "Work Item ID (e.g. 12345) and short description of the work item"
---
```

- Observe the main agent is not progressing due to the missed sub agent notifications

### Expected behavior

Orchestration pattern works

### Additional context

- Ubuntu Linux
- Related issue that should be reopened: #2051

Start by reproducing the orchestration flow described in the issue on GitHub Copilot CLI 1.0.34, then review related issue #2051 for prior context. Done means the main agent receives sub-agent completion notifications and continues the orchestration pattern in the CLI.

索引モデルが issue の本文から書いたものです。