feat(transport): add STDIO transport support

[CalvinAllen]

Epic: STDIO Transport Support (Broker + Shim Architecture)

Add STDIO transport support and multi-instance VS support via a central Broker and lightweight STDIO Shim.

Summary

A central Broker.exe manages multiple VS instances — each Server.exe registers with the broker on startup, and the broker transparently proxies MCP tool calls to the correct instance. A lightweight Shim.exe bridges MCP STDIO transport to the broker's HTTP/SSE endpoint, enabling Claude Desktop and Claude Code integration. Existing Server.exe and VS Extension core plumbing remain largely unchanged.

Key insight: Instead of inverting the pipe architecture (original plan), a broker + shim approach solves both STDIO transport (#18) and multi-instance support (#54) with minimal changes to existing code.

Architecture

```
┌──────────────┐ ┌──────────────┐
│ VS Instance 1│ │ VS Instance 2│
│ Server.exe │ │ Server.exe │
│ (port: auto) │ │ (port: auto) │
└──────┬───────┘ └──────┬───────┘
│ register │ register
▼ ▼
┌─────────────────────────────────────┐
│ Broker.exe │
│ HTTP/SSE on port 5100 (default) │
│ MCP tools: list_instances, │
│ select_instance │
│ Transparent proxy to selected │
│ instance's Server.exe │
│ Health checks (15s interval) │
│ 30s shutdown timer after last VS │
└──────────┬──────────────────────────┘
│
┌─────┴──────┐
▼ ▼
HTTP client Shim.exe
(:5100/sse) (STDIO → :5100)
▲
│ spawns
Claude Desktop/Code
```

Design Decisions

Decision	Choice
Broker binary	Separate Broker.exe
Instance identity	Solution path + directory + display name + PID
Routing	One connection = one instance
Instance matching	Auto-match by working directory, fallback to list
Broker lifecycle	Timeout 30s after last VS, then exit
Server↔Broker comm	Each Server.exe keeps HTTP, broker proxies
Broker port	Default 5100, configurable in VS settings
Discovery	MCP tools on broker (list_instances, select_instance)
Proxy mode	Transparent — broker re-exposes instance tools
Port allocation	Server.exe binds port 0, reports actual port to broker
No-selection behavior	Error with instance list
No-solution VS	Registers with null solution info
Standalone mode	Setting to disable broker (preserves current behavior)
Shim	Separate Shim.exe, bridges STDIO to broker HTTP

Sub-Issues

Phase 1: Shared

• feat(shared): add broker communication models and RPC contract update #58 - feat(shared): add broker communication models and RPC contract update

Phase 2: Broker

• feat(broker): create Broker.exe with instance registry and MCP proxy #59 - feat(broker): create Broker.exe with instance registry and MCP proxy

Phase 3: Server Changes

• feat(server): add port 0 binding and broker registration #60 - feat(server): add port 0 binding and broker registration

Phase 4: Extension Changes

• feat(extension): add broker lifecycle management and solution change forwarding #61 - feat(extension): add broker lifecycle management and solution change forwarding

Phase 5: Shim

• feat(shim): create STDIO-to-broker bridge executable #62 - feat(shim): create STDIO-to-broker bridge executable

Phase 6: Packaging

• build(vsix): package Broker.exe and Shim.exe with extension #63 - build(vsix): package Broker.exe and Shim.exe with extension

Phase 7: Documentation

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

Dependencies

```
#58 (Shared) ──► #59 (Broker) ──┬──► #61 (Extension)
│
#60 (Server) ───┘

             #62 (Shim) — parallel with Phase 3/4
             
             #63 (Packaging) — after all projects exist
             
             #26 (Docs) — last

```

Also Addresses

• Support for multiple instances of Visual Studio #54 - Support for multiple instances of Visual Studio
• no Https posible #55 - no Https posible (STDIO bypasses HTTP/HTTPS for local use)

Acceptance Criteria

• Broker manages multiple VS instance registrations
• Transparent proxy routes tool calls to selected instance
• STDIO transport works via Shim for Claude Desktop/Code
• HTTP/SSE transport works via broker for other clients
• Standalone mode (no broker) preserves current behavior
• Health checks detect and deregister crashed instances
• Solution changes reflected in broker registry
• All three executables ship in the VSIX
• Documentation updated