feat(transport): add STDIO transport support

[CalvinAllen]

Epic: STDIO Transport Support

Add STDIO transport support alongside existing HTTP/SSE, allowing Claude Desktop to spawn the MCP server directly.

Summary

The architecture inverts the pipe relationship: in STDIO mode, Server.exe becomes the pipe server and VS Extension connects as a client.

Key constraint: Per-user installation is retained (no admin required), so STDIO mode requires VS to already be running with the extension loaded.

Architecture

Aspect	HTTP/SSE (Current)	STDIO (New)
Who spawns Server.exe	VS Extension	Claude Desktop
Pipe server	VS Extension	Server.exe
Pipe client	Server.exe	VS Extension
MCP transport	HTTP on port 5050	stdin/stdout
VS requirement	Spawns with server	Must be running first

STDIO Flow

1. User opens Visual Studio (extension loads, watches for STDIO pipes)
2. Claude Desktop spawns Server.exe --transport stdio
3. Server.exe creates named pipe server (vsmcp-stdio)
4. VS Extension detects pipe, connects as client
5. Server.exe communicates with Claude via STDIO

Sub-Issues

Phase 1: Shared

• feat(shared): add McpConstants for STDIO pipe name #19 - feat(shared): add McpConstants for STDIO pipe name

Phase 2: Server Changes

• feat(server): add RpcServer for STDIO pipe server mode #20 - feat(server): add RpcServer for STDIO pipe server mode
• feat(server): add --transport option and STDIO mode to Program.cs #21 - feat(server): add --transport option and STDIO mode to Program.cs

Phase 3: Extension Changes

• feat(extension): add IRpcClient interface and RpcClient for pipe client mode #22 - feat(extension): add IRpcClient interface and RpcClient for pipe client mode
• feat(extension): add StdioPipeWatcher service #23 - feat(extension): add StdioPipeWatcher service
• feat(extension): update MCPServerPackage for connection mode tracking #24 - feat(extension): update MCPServerPackage for connection mode tracking
• feat(extension): update ServerCommands for connection mode #25 - feat(extension): update ServerCommands for connection mode

Phase 4: Documentation

• docs: update README with STDIO transport configuration #26 - docs: update README with STDIO transport configuration

Dependencies

#19 (Constants) ─┬─► #20 (Server RpcServer) ─► #21 (Program.cs)
                 │
                 └─► #22 (Extension RpcClient) ─► #23 (PipeWatcher) ─► #24 (Package) ─► #25 (Commands)
                                                                                              │
                                                                                              ▼
                                                                                        #26 (Docs)

Claude Desktop Configuration

{
  "mcpServers": {
    "visual-studio": {
      "command": "path/to/CodingWithCalvin.MCPServer.Server.exe",
      "args": ["--transport", "stdio"]
    }
  }
}

Note: User must have Visual Studio open with the MCP Server extension loaded before starting Claude Desktop.

Acceptance Criteria

• STDIO transport works when VS is already open
• HTTP/SSE mode unchanged and fully functional
• Clear error messages when VS is not running
• Mode conflict handling (HTTP takes priority)
• Documentation updated