Local MCP servers flash terminal window on Windows #29532

Describe the bug
When launching local MCP servers on Windows, a console window briefly flashes on screen every time the MCP server process is spawned or reconnected. This is very distracting during normal usage.

To Reproduce

1. Configure any local MCP server in opencode.json
2. Start opencode on Windows
3. Observe terminal windows flashing when MCP server connects

Expected behavior
No visible terminal window should appear for background MCP server processes.

Root cause
When spawning child processes on Windows, opencode does not set the CREATE_NO_WINDOW (0x08000000) creation flag or HideWindow: true in syscall.SysProcAttr.

Suggested fix
In the Go code that spawns local MCP server processes, add HideWindow: true to syscall.SysProcAttr on Windows:

cmd := exec.Command(...)
cmd.SysProcAttr = &syscall.SysProcAttr{
    HideWindow: true, // Windows only
}

Environment

• OS: Windows 11
• opencode version: latest