Expose per-model token usage (model_usage array) in the status line payload

## Feature request: expose per-model token usage in the status line payload

### Context

The configurable status line (`statusLine.command`) receives a JSON payload on stdin. Token usage is currently exposed two ways:

```js
context_window: {
total_input_tokens, // ← SUM across ALL models used in the session
total_output_tokens, // ← same
total_cache_read_tokens, // ← same
total_cache_write_tokens, // ← same
total_reasoning_tokens, // ← same
...
current_usage: { // ← ONLY for the currently selected model
input_tokens,
output_tokens,
cache_creation_input_tokens,
cache_read_input_tokens
}
}
```

The runtime tracks usage per model in its internal `modelMetrics` map (this is visible in `session.shutdown` events and surfaced by `/usage`), but **only the cross-model sum and the currently-selected model's slice are exposed** to the status line script.

### Problem

This makes it impossible for a status line script to compute an accurate session cost when the user switches models mid-session (e.g. starts in GPT-5.5, `/model` to Claude Opus 4.7). The script can only apply the currently selected model's rates to all cumulative tokens — which under- or over-estimates depending on which model is pricier.

Concrete example: spend 1M output tokens on GPT-5.5 ($30/M output), then switch to Opus 4.7 ($25/M output). True cost is $30. A status line script can only see "current model = Opus, total_output = 1M" and reports $25. Off by 20%.

### Proposal

Expose the per-model breakdown alongside the existing totals:

```js
context_window: {
total_input_tokens,
total_output_tokens,
total_cache_read_tokens,
total_cache_write_tokens,
total_reasoning_tokens,
...

// NEW: array of per-model usage slices
model_usage: [
{
model_id: "gpt-5.5",
model_display_name: "GPT-5.5",
input_tokens: 700000,
output_tokens: 700000,
cache_read_tokens: 700000,
cache_write_tokens: 35000,
reasoning_tokens: 0,
requests: 12
},
{
model_id: "claude-opus-4.7",
model_display_name: "Claude Opus 4.7",
input_tokens: 300000,
output_tokens: 300000,
cache_read_tokens: 300000,
cache_write_tokens: 15000,
reasoning_tokens: 5000,
requests: 4
}
]
}
```

This data already exists internally as `modelMetrics` and is exactly what `/usage` displays today. Exposing it would be a one-line payload change.

### Use case

Any status line script that surfaces session cost in USD (using the published model pricing) would become correct for multi-model sessions. The script just iterates `model_usage`, looks up each model's rates, and sums.

### Current workaround (and why it's bad)

Parsing `~/.copilot/session-state//events.jsonl` for `assistant.message.outputTokens` + `model` per turn — fragile (race conditions with the writer), incomplete (input/cache tokens are NOT logged per call, only aggregated at `session.shutdown`), and forces every status line author to reinvent the same incremental-parsing + caching machinery.

### Related

Companion request for `total_files_modified`: #3404