feat(shared): add McpConstants for STDIO pipe name

[CalvinAllen]

Parent Epic

Part of #18 - STDIO Transport Support

Description

Create a shared constants file for the well-known STDIO pipe name.

Implementation

New File: src/CodingWithCalvin.MCPServer.Shared/Constants.cs

namespace CodingWithCalvin.MCPServer.Shared;

public static class McpConstants
{
    public const string StdioPipeName = "vsmcp-stdio";
}

Acceptance Criteria

• Constants.cs created in Shared project
• StdioPipeName constant defined
• Both Server and Extension projects can reference it