Breaking Change: .vscode/mcp.json is no longer supported

### Describe the bug

https://github.com/github/copilot-cli/issues/39 requested that VSCode's mcp.json support be added, but this capability was recently removed. This means we need to maintain multiple sets of MCP.json files to maintain parity between our CLI users and our VSCode Copilot users.

### Affected version

1.0.37

### Steps to reproduce the behavior

1. Start github copilot CLI in a repo with .vscode/mcp.json
2. /mcp
3. You won't see the MCP servers

### Expected behavior

We should have the MCP servers that are defined in .vscode/mcp.json

### Additional context

_No response_

Start by reproducing the issue with a .vscode/mcp.json file, launch GitHub Copilot CLI in that repository, and run /mcp. Trace how the CLI discovers MCP servers and compare that behavior with the expected servers defined in .vscode/mcp.json; done means those servers appear in /mcp for affected users.

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