Windows: plugin update fails with "Access is denied (os error 5)" while VS Code is running (Copilot extension holds watcher handles on installed-plugins)

### Describe the bug

`copilot plugin update` (and the same flow triggered from the GitHub Copilot desktop app) fails on Windows with:

```
Failed to update plugin: Error: Failed to install plugin: Error: Access is denied. (os error 5)
```

The git fetch/checkout of the marketplace clone succeeds every time (the clone under `%LOCALAPPDATA%\copilot\marketplaces\...` ends up at the target commit, working tree clean). The failure happens in the final step that replaces `~\.copilot\installed-plugins\\`.

Root cause (verified with a system-wide handle enumeration via `NtQuerySystemInformation(SystemExtendedHandleInformation)`): **VS Code's Copilot extension holds directory watcher handles** (access mask `0x100081`, i.e. `ReadDirectoryChangesW`-style) on the installed plugin folder itself and its `hooks`, `agents`, and `skills` subdirectories:

```
pid=15064 handle=0x424 access=0x100081 path=\?\C:\Users\\.copilot\installed-plugins\\al-agentic-dev
pid=15064 handle=0x65c access=0x100081 path=...\al-agentic-dev\hooks
pid=15064 handle=0x678 access=0x100081 path=...\al-agentic-dev\agents
pid=15064 handle=0x67c access=0x100081 path=...\al-agentic-dev\skills
```

(pid 15064 = `Code.exe --type=utility --utility-sub-type=node.mojom.NodeService` — the VS Code file-watcher host.)

On Windows, a directory that has an open watcher handle cannot be renamed/replaced, so the installer's folder swap fails with `ERROR_ACCESS_DENIED (5)`. Closing VS Code releases the handles and the identical update command then succeeds immediately (v2.5.0 → v3.0.0 in my case).

This is very confusing for users: restarting the Copilot desktop app doesn't help, and even a reboot doesn't help if VS Code relaunches before the update is retried. The error message gives no hint that another application is holding the folder.

### Affected version

GitHub Copilot CLI 1.0.70 (also reproduced through GitHub Copilot desktop app 1.0.21, which calls the same installer via `plugins.update`)

### Steps to reproduce the behavior

1. On Windows, install a plugin from a marketplace, with the VS Code GitHub Copilot extension active (so it watches `~\.copilot\installed-plugins\...`).
2. With VS Code still running, run `copilot plugin update @` (or click Update in the desktop app's plugin settings).
3. Error: `Failed to install plugin: Error: Access is denied. (os error 5)`. The installed plugin stays at the old version.
4. Close VS Code, run the same command → update succeeds.

### Expected behavior

The update should succeed while VS Code is running — e.g. by copying the new version into place file-by-file instead of renaming/removing the directory root, by retrying with backoff, or by staging to a sibling dir and swapping with `FILE_RENAME_FLAG_POSIX_SEMANTICS`. Failing that, the error message should name the process holding the directory (the Restart Manager API can identify it) so users don't reach for a reboot.

### Additional context

- Operating system: Windows 11 Pro 10.0.26200, x86_64
- Diagnosis details: per-file probes all pass (no file locks, no read-only attributes, ACLs fine — Restart Manager reports nothing because it doesn't surface directory watcher handles); only the rename of the plugin folder root fails. Bisected by rename-probing every subdirectory: only the root and the three watched subdirs are held.
- The plugin in question ships `hooks/`, `agents/`, `skills/` and an `extensions/` canvas extension, but the bug is independent of plugin content — any plugin folder watched by VS Code's Copilot extension will hit it.

Bắt đầu tại điểm vào `copilot plugin update` và lần theo bước của trình cài đặt Windows thay thế thư mục plugin đã cài đặt. Tái hiện lỗi khi VS Code đang chạy và tiện ích Copilot của nó đang hoạt động; được xem là hoàn tất khi quá trình cập nhật thành công mà không cần đóng VS Code, hoặc lỗi xác định được tiến trình đang giữ thư mục.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.