1.0.48-0: win32-arm64 prebuilds ships x64 runtime.node instead of ARM64

## Description

On a fresh install of Copilot CLI v1.0.48-0 on Windows ARM64, launching copilot fails with:

Native addon "runtime" not found for win32-arm64.

## Root Cause

The universal package at ~/.copilot/pkg/universal/1.0.48-0/prebuilds/ ships runtime.node for all platforms except win32-arm64:

prebuilds/darwin-arm64/runtime.node OK
prebuilds/darwin-x64/runtime.node OK
prebuilds/linux-arm64/runtime.node OK
prebuilds/linux-x64/runtime.node OK
prebuilds/win32-x64/runtime.node OK
prebuilds/win32-arm64/runtime.node MISSING

There is a file at prebuilds/win32-arm64/win32-x64/runtime.node, but it is an x64 binary. Placing it at the expected path produces "is not a valid Win32 application" because the ARM64 Node process cannot load an x64 native addon.

## Comparison with v1.0.47

Version 1.0.47 shipped the correct ARM64 runtime at a different path: native/runtime/runtime.win32-arm64-msvc.node. This file works correctly when copied to prebuilds/win32-arm64/runtime.node in v1.0.48-0.

## Workaround

Copy the ARM64 runtime from v1.0.47 (if still cached locally):

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

- OS: Windows 11 ARM64
- Copilot CLI: 1.0.48-0 (installed via winget)
- Architecture: ARM64 native (copilot.exe is ARM64)

首先比較 v1.0.48-0 和 v1.0.47 的 package 內容，尤其是 prebuilds/win32-arm64/runtime.node 和 native/runtime/runtime.win32-arm64-msvc.node。追蹤 universal package 的 prebuilds 如何組裝，並驗證全新的 Windows ARM64 安裝在預期路徑包含 ARM64 runtime，且能夠成功啟動。

由索引模型根據 Issue 內容生成。