Expose total_files_modified in the status line payload

## Feature request: expose `total_files_modified` in the status line payload

### Context

The configurable status line (`statusLine.command`) receives a JSON payload on stdin with a `cost` object that currently includes:

```js
cost: {
total_api_duration_ms,
total_lines_added,
total_lines_removed,
total_duration_ms,
total_premium_requests
}
```

`total_lines_added` / `total_lines_removed` are great for showing a `+12/-3`‑style diff summary, but there's no companion field for the **number of files touched** in the session.

### Proposal

Add `total_files_modified: number` (the count, not the array) to the `cost` payload sent to the status line script. The runtime already tracks `codeChanges.filesModified` internally (it surfaces in `/usage` and in `session.shutdown` events), so this is essentially exposing existing state.

```js
cost: {
total_api_duration_ms,
total_lines_added,
total_lines_removed,
total_files_modified, // ← new
total_duration_ms,
total_premium_requests
}
```

### Use case

Status line scripts can render a more complete change summary, e.g.:

```
Context 25k/100k [bar] 25% · ↑112k ⊚80k ↓5k ($0.35) · +42/-13 in 4 files
```

This is the natural complement to the line‑count fields and matches how most diff tools summarize changes (lines + file count).

### Why a count and not the list

Sending the full path list would bloat the payload on long sessions and leak filesystem context unnecessarily. A simple integer count is enough for the status‑line use case.

### Alternative considered

Reading `~/.copilot/session-state//events.jsonl` from the script — rejected as fragile (race conditions with the writer, 200 ms refresh budget, parsing a growing JSONL on every tick).

Start at the statusLine.command payload construction and trace how the existing cost fields are assembled from the runtime's codeChanges.filesModified state. Add the count alongside the other cost fields, then verify that the status line receives total_files_modified as a number and that the existing payload fields remain unchanged.

Written by the indexing model from the issue text.