[Windows] CMD windows flash on screen when MCP servers start

### Describe the bug

On Windows, every time a new Copilot CLI session starts, multiple cmd.exe windows briefly flash on screen (pop up then close). This happens because MCP servers are launched via cmd.exe /c agency mcp , and cmd.exe opens a visible console window by default on Windows.

With multiple concurrent sessions and 4-5 MCP servers each (sharepoint, teams, mail, calendar, workiq), this results in **15+ CMD windows flashing** in quick succession — very disruptive to the user experience.

### Steps to reproduce

1. Run Copilot CLI on Windows with MCP servers configured (e.g., sharepoint, teams, mail, calendar)
2. Start a new session
3. Observe multiple cmd.exe windows briefly appearing and disappearing

### Evidence from process monitoring

`
[12:48:28] cmd.exe /c agency mcp sharepoint <- parent: copilot.exe
[12:48:28] cmd.exe /c agency mcp teams <- parent: copilot.exe
[12:48:28] cmd.exe /c agency mcp mail <- parent: copilot.exe
[12:48:29] cmd.exe /c agency mcp calendar <- parent: copilot.exe
`
Repeated for each concurrent session (3 sessions = 12+ CMD popups within 5 seconds).

### Expected behavior

MCP server processes should start without visible console windows. On Windows, this can be achieved by using CREATE_NO_WINDOW or DETACHED_PROCESS process creation flags, or by launching via conhost.exe --headless, or by using start /b instead of cmd.exe /c.

### Environment

- OS: Windows 11
- Copilot CLI: installed via WinGet (`GitHub.Copilot`)
- MCP servers: sharepoint, teams, mail, calendar, workiq (via Agency Cowork)