Show per-subagent AI credit usage breakdown in /usage

### Describe the feature or problem you'd like to solve

Copilot CLI displays cumulative AI-credit usage for the session, including 4 usage by the main agent, subagents, and background operations. However, it 5 does not show how those credits are distributed among individual agent 6 invocations. 7   8 This makes it difficult to identify expensive agents, compare custom-agent 9 models, and optimize multi-agent workflows.

### Proposed solution

Extend `/usage` with an optional itemized breakdown showing:

- Main-agent usage
- Each subagent invocation
- Custom-agent name
- Model used
- AI credits consumed
- Background operations such as context compaction
- Total session usage

### Example prompts or workflows

### Use case

When a parent agent delegates investigation, implementation, review, and
testing to several subagents, users need to understand which invocation or
configured model accounts for most of the session cost.

### Example

```
Main agent 4.20 AIC
Explore #1 — architecture 1.15 AIC
ios-implementer #1 7.80 AIC
Task #1 — tests 0.65 AIC
Context compaction 0.30 AIC
--------------------------------------------
Session total 14.10 AIC
```

Possible commands:

`/usage`
`/usage --by-agent`
`/usage --detailed`

### Additional context

_No response_

/usage コマンドと、issue に記載されている既存のセッション クレジットの集計から始めます。main-agent、subagent、custom-agent、model、background-operation の使用量がどのように記録されるかを追跡し、そのうえで提案されているコマンド形式に対して表示する内訳と合計を定義します。/usage で呼び出しごとの詳細を表示しつつ、セッションの累積合計を維持できれば完了です。

索引モデルが issue の本文から書いたものです。