feat: add stdio-proxy command for HTTP-to-stdio MCP bridging

[wasaga]

Summary

• Add new stdio-proxy command that launches a stdio MCP server subprocess and exposes it via HTTP streaming
• Subprocess lifecycle management with graceful shutdown and crash detection
• Automatic capability discovery and forwarding (tools, prompts, resources, resource templates)
• Request/response logging with terminal-aware formatting (text for TTY, JSON for daemon)
• Concurrent request handling with shared subprocess model
• Health checking with 503 responses when subprocess unavailable
• Full bidirectional MCP support:
  • Notification handlers (logging, tool/prompt/resource list changes)
  • Sampling/CreateMessage (server can request LLM completions from client)
  • Elicitation (server can request user input from client)

Test plan

• Unit tests for tools, prompts, resources (7 tests)
• Unit tests for bidirectional features (5 tests):
  • Logging notifications
  • Sampling with and without handler
  • Elicitation with and without handler
• Manual test with real stdio MCP server
• Test subprocess crash handling

Usage

# Start proxy for a stdio MCP server
mcp-servers stdio-proxy -addr localhost:8080 -- /path/to/stdio-server

# With debug logging
mcp-servers stdio-proxy -addr localhost:8080 -log-level debug -- /path/to/stdio-server