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_

Start at the /usage command and the existing session credit accounting described in the issue. Trace how main-agent, subagent, custom-agent, model, and background-operation usage are recorded, then define the displayed breakdown and totals for the proposed command forms. Done means /usage can show per-invocation details while preserving the cumulative session total.

Written by the indexing model from the issue text.