Bug: /copy fails with "Connection refused" — snap package missing x11/wayland plug

### Describe the bug

On Linux, the `/copy` command in GitHub Copilot CLI fails with:

`Failed to copy to clipboard: Error: Failed to create clipboard context: Connection refused (or error 111)`

### Root Cause
The Linux distribution of Copilot CLI is packaged as a **snap**, and the snap's declared interfaces do not include `x11` or `wayland`. Clipboard access on Linux requires a direct connection to the X11 (or Wayland) display server socket, which snap's confinement blocks unless the snap explicitly declares and connects one of those interfaces. Since neither is declared, there is no `snap connect` command available to fix this — the plug simply doesn't exist in the snap's metadata.

### Affected version

1.0.70 (Install method: snap `/snap/copilot-cli/53/bin/copilot`)

### Steps to reproduce the behavior

1. Install/run GitHub Copilot CLI via the snap package on a Linux desktop with an active X11 session.
2. Start an interactive session and let the assistant produce any response.
3. Run `/copy` to copy the last response to the system clipboard.

### Expected behavior

The last assistant response is copied to the system clipboard, as documented in `/help`:

`/copy - Copy the last response to the clipboard`

### Additional context

#### OS/Environment
- **OS/session:** Ubuntu-based Linux desktop, X11 session (`XDG_SESSION_TYPE=x11`, `DISPLAY=:1`)
- **Desktop Environment:** GNOME desktop (`gnome-terminal-server`)
- **Streaming Protocol:** Amazon DCV/WSP streaming protocol (AWS WorkSpaces), accessed via the AWS WorkSpaces client on macOS (Apple Silicon MacBook Pro)
- **Terminal emulator:** gnome-terminal

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#### Diagnostic Steps & Evidence

1. **Clipboard Works Separately:** Copy/paste between other applications in the same session (terminal, text editor, browser) works correctly. This rules out broken X11 sessions, window manager clipboard mechanisms, or remote-desktop bridging problems. The issue is unique to the Copilot CLI process.
2. **Snap Confinement Restrictions:** Running commands inside the CLI's shell tool environment yields permission blocks:
- `/usr/bin/snap` returns `Permission denied` (exit code 126).
- Running a normal executable (`venv/bin/python`, mode `0755` owned by current user) returns `Permission denied` (exit code 126).
- `xset q` reports the X server as unreachable when run from inside the CLI's shell tool.
3. **Missing Plugs:** `snap connections copilot-cli` shows the snap only requests `desktop` / `desktop-legacy` but lacks `x11` and `wayland`:

首先定位 snap 软件包的元数据，并检查 `snap connections copilot-cli` 暴露的接口；该 issue 指出缺少 `x11` 和 `wayland` plugs 是疑似原因。在 Linux 桌面会话中复现 `/copy`，然后验证 snap 是否暴露了所需的显示访问权限，以及该命令是否成功复制最后一条响应。

由索引模型根据 Issue 内容生成。