Disabled and then removed MCP servers remain in `disabledMcpServers`

### Short summary

Disabled and then removed MCP servers remain in `disabledMcpServers`

### Affected version or release

v0.2.7

### Installation context

_No response_

### What happened?

Disabling an MCP server and then deleting it keeps the disabledMcpServers state in ~/.copilot/settings.json

This can be problematic when an mcp server defined in the plugin shares the same name. For example I manually added a `seismic` mcp server, disabled it, deleted it, and then added a plugin that references a `seismic` mcp server. The skill in the plugin failed to work because it was marked as disabled.

### Steps to reproduce

1. Add mcp server
2. disable mcp server
3. delete mcp server
4. Note output of `cat ~/.copilot/settings.json | jq .disabledMcpServers`

### Expected behavior

disabledMcpServers is cleared when the mcp server is deleted.

### Additional context

_No response_