WebSocket transport for persistent agent attachment

[rodaddy]

Summary

Add WebSocket transport so mcp2cli can act as a persistent tool server that agents connect to for real-time tool discovery, schema introspection, and streamed results.

Motivation

• HTTP/SSE gives request-response access to remote MCP servers
• WebSocket enables persistent connections where agents attach once and stream tool calls bidirectionally
• Similar to how OpenAI Codex connects to tool servers -- agents get real-time discovery without pre-loading schemas
• Natural evolution toward the LXC gateway vision but accessible to any agent runtime

Proposed Behavior

1. mcp2cli serve --ws starts a WebSocket server
2. Agents connect via ws://localhost:<port>
3. Over WS, agents can:
   • Discover all registered services and tools (unified schema across all backends)
   • Invoke tools with streamed results
   • Subscribe to health/status events
4. mcp2cli multiplexes across stdio, HTTP, and future backends transparently

Transport Evolution

Transport	Use Case	Status
stdio	Local MCP processes	shipped
HTTP/SSE	Remote servers, LXC gateway	PR #1
WebSocket	Persistent agent attachment, streaming	this issue