plugin install fails with Access is denied (os error 5) on Windows for all sources

### Bug Description
`copilot plugin install` fails 100% of the time on Windows 11 with:

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

It fails for a marketplace source (`azure@azure-skills`), a direct GitHub repo, **and** a local directory source — so it is not specific to one plugin or fetch method.

### Version
- GitHub Copilot CLI: **1.0.71** (also seen on 1.0.70)
- OS: **Windows 11 Enterprise 10.0.26200, x64**
- git: 2.50.1.windows.1

### Steps to reproduce
1. `copilot plugin marketplace add microsoft/azure-skills`
2. `copilot plugin install azure@azure-skills`
3. Fails with `Access is denied. (os error 5)`

### Expected behavior
Plugin installs successfully.

### Actual behavior
Install aborts. It creates an **empty** target folder under `~/.copilot/installed-plugins/` (e.g. `azure-skills` and `_direct`) then fails on its first file operation, before any file is written.

### Investigation / ruled out
- **Not permissions/sandbox:** fails identically when run directly as the real user with Full Control over the entire `~/.copilot` tree.
- **Not Controlled Folder Access:** `Get-MpPreference` shows `EnableControlledFolderAccess = 0`.
- **Not git:** `git clone --depth 1 https://github.com/microsoft/azure-skills.git` succeeds (1552 files).
- **Not a lock:** the leftover target folder can be freely renamed/deleted.
- Plain create/write/delete under `installed-plugins` and `%TEMP%` all succeed as the same user.
- Fails for both remote and local sources, immediately after the target dir is created.
- No detail is written to `~/.copilot/logs`; the message is stderr-only and double-wrapped, hiding the offending path.

### Suspected cause
A native filesystem op in the install/cache pipeline (likely `remove_dir_all`/rename/copy) returning Windows `ERROR_ACCESS_DENIED` — a common Rust-on-Windows failure against read-only files (e.g. git pack objects). **Surfacing the actual path in the error would greatly help.**

### Workaround
`copilot --plugin-dir ` loads the plugin fine, confirming only the install/copy path is broken.

Reproduziere den Fehler mit den Installationsbefehlen für marketplace, direktes Repository und lokales Verzeichnis und verfolge anschließend die Installations-/Cache-Pipeline direkt nachdem das Zielverzeichnis erstellt wurde. Identifiziere die erste Dateisystemoperation, die ERROR_ACCESS_DENIED zurückgibt, und gib ihren Pfad aus; abgeschlossen ist die Aufgabe, wenn die Plugin-Installation unter Windows für jede Quelle erfolgreich ist und der Fehler bei einem Fehlschlag den problematischen Pfad angibt.

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