Marketplace: cannot remove registered plugin, because it's "not registered"

### Describe the bug

Bug: cannot remove registered plugin, the outputs speak for themselves. Note the contradictions: plugin cannot be installed because it's already registered, plugin cannot be removed because it's not registered. It is installed, it is registered, and it can be used.

```
C:\dev\tmp\ct-quickshare>copilot plugin marketplace add microsoft/github-copilot-modernization
Failed to add marketplace: Error: Marketplace "github-copilot-modernization" already registered

C:\dev\tmp\ct-quickshare>copilot plugin marketplace remove microsoft/github-copilot-modernization
Failed to remove marketplace: Error: Marketplace "microsoft/github-copilot-modernization" is not registered
```

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Start with the `copilot plugin marketplace add` and `remove` commands shown in the issue and compare how each handles `microsoft/github-copilot-modernization`. Reproduce the reported errors and trace the relevant CLI entry points. Done means a marketplace that reports as already registered can be removed successfully.

索引モデルが issue の本文から書いたものです。