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.

Commencez par examiner le schéma de .vscode/mcp.json et le chemin de configuration .github/extensions/ existant de l’application ; comparez la façon dont sont représentés les serveurs HTTP et stdio, les en-têtes, cwd et la substitution de variables. Le travail est considéré comme terminé lorsqu’un .vscode/mcp.json à la racine du dépôt est découvert et que les deux types de serveurs fonctionnent sans configuration en double.

Rédigé par le modèle d'indexation à partir du texte de l'issue.