UI shows background agents as 'running' after they have completed

## Description

The terminal UI displays background agents/tasks as still running (e.g., "10 background tasks running") even after all agents have completed and their results have been successfully retrieved via `read_agent`.

## Steps to Reproduce

1. Launch multiple background agents (e.g., 9 bitsmith agents in parallel)
2. Wait for all agents to complete (receive completion notifications)
3. Retrieve results from each with `read_agent`
4. Observe the UI still shows tasks as "running"

## Expected Behavior

Once an agent completes and its results are read, it should no longer appear as a running task in the UI.

## Actual Behavior

The UI continues to show completed agents as running. `list_agents` correctly returns no running agents, but the UI indicator doesn't update.

## Environment

- Copilot CLI version: 1.0.49
- OS: macOS (Darwin)
- Model: Claude Opus 4.6

Start at the terminal UI's background-task indicator and trace how its state is updated after the `read_agent` entry point retrieves results. Compare that state with the empty result from `list_agents` when all agents have completed. Done means the UI no longer counts completed agents as running after their results are read.

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