Feature Request: Support .vscode/mcp.json for MCP server discovery

### Feature summary

VS Code's Copilot extension supports .vscode/mcp.json as a project-level configuration file that defines MCP (Model Context Protocol) servers. This allows teams to check in a shared set of MCP servers that all contributors automatically pick up.

### What problem are you trying to solve?

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## Feature Request: Support `.vscode/mcp.json` for MCP server discovery

### Problem

VS Code's Copilot extension supports `.vscode/mcp.json` as a project-level configuration file that defines MCP (Model Context Protocol) servers. This allows teams to check in a shared set of MCP servers that all contributors automatically pick up.

However, the **Copilot CLI app** (GitHub's standalone desktop agent) does not recognize `.vscode/mcp.json`. This means teams that have already configured their MCP servers for VS Code must separately configure them again using the app's extension system (`.github/extensions/`), leading to duplicated configuration and maintenance burden.

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### Proposed solution

### Proposed Solution

The Copilot CLI app should discover and load MCP servers defined in `.vscode/mcp.json` at the repository root, using the same schema that VS Code supports. This includes:

- **HTTP-type servers** (`"type": "http"` with a `"url"` and optional `"headers"`)
- **Stdio-type servers** (`"type": "stdio"` with `"command"`, `"args"`, and optional `"cwd"`)
- **Variable substitution** (e.g., `${userHome}`, `${workspaceFolder}`)

### Benefits

1. **Single source of truth** — one config file works across VS Code and the Copilot CLI app
2. **Zero onboarding friction** — new team members get MCP servers automatically without extra setup
3. **Ecosystem consistency** — aligns with the broader MCP ecosystem's convention for project-level config

### Alternatives Considered

- `.github/extensions/` — requires a completely different format (executable scripts), doesn't support HTTP-type servers declaratively, and isn't portable to VS Code
- Manual per-user setup — doesn't scale for teams

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

### Additional Context

The `.vscode/mcp.json` format is already a de facto standard for project-scoped MCP server configuration. Supporting it in all GitHub Copilot surfaces would reduce fragmentation and improve the developer experience for teams using multiple Copilot interfaces.

Start by reviewing the .vscode/mcp.json schema and the app's existing .github/extensions/ configuration path; compare how HTTP and stdio servers, headers, cwd, and variable substitution are represented. Done means a repository-root .vscode/mcp.json is discovered and both server types work without duplicate setup.

Written by the indexing model from the issue text.