runtime.node missing from prebuilds/win32-arm64 in 1.0.48-0

## Description

On Windows ARM64, Copilot CLI 1.0.48-0 fails immediately with:

`
✗ Execution failed: Error: Native addon "runtime" not found for win32-arm64.
`

## Root Cause

The `prebuilds/win32-arm64/` directory in the 1.0.48-0 package is missing `runtime.node`. Every other platform has it:

- `prebuilds/win32-x64/` → has `runtime.node` ✅
- `prebuilds/darwin-arm64/` → has `runtime.node` ✅
- `prebuilds/darwin-x64/` → has `runtime.node` ✅
- `prebuilds/linux-arm64/` → has `runtime.node` ✅
- `prebuilds/linux-x64/` → has `runtime.node` ✅
- `prebuilds/win32-arm64/` → **missing** ❌

Contents of `prebuilds/win32-arm64/` in 1.0.48-0:
- `computer.node`
- `conpty_console_list.node`
- `conpty.node`
- `keytar.node`

Compare with `prebuilds/win32-x64/`:
- `computer.node`
- `conpty_console_list.node`
- `conpty.node`
- `keytar.node`
- `runtime.node` ← this one is missing from arm64
- `win32-native.node`

## Note

This is distinct from #3298 (missing `index.js` loader). In 1.0.47 the binary existed at `native/runtime/runtime.win32-arm64-msvc.node` — it appears the migration to the new `prebuilds/` layout in 1.0.48-0 dropped it for arm64 only.

## Workaround

Copy the runtime binary from the 1.0.47 package:

`powershell
Copy-Item "~\.copilot\pkg\universal\1.0.47\native\runtime\runtime.win32-arm64-msvc.node" "~\.copilot\pkg\universal\1.0.48-0\prebuilds\win32-arm64\runtime.node"
`

## Environment

- Windows 11 ARM64
- Copilot CLI 1.0.48-0 (installed via winget, self-updated)