Add HTTP/SSE transport support — gateway currently limited to stdio downstream servers

[himerus]

Summary

The MCP gateway only supports stdio transport for downstream server connections. HTTP and SSE downstream MCP servers cannot be proxied at all.

Problem

Many MCP servers run as HTTP endpoints (including cloud-hosted and team-shared servers). The stdio-only limitation means reagent cannot proxy:

• Any remote MCP server
• Any team-shared MCP server
• Any cloud-hosted MCP server (e.g. Cloudflare Workers, Vercel Edge)

Proposed Solution

Implement HTTP/SSE client transport in client-manager.ts so downstream servers running as HTTP endpoints can be connected. Configuration in gateway.yaml:

```yaml
servers:
my-remote-server:
transport: sse
url: https://my-mcp-server.example.com/sse
```

Reference: MCP specification HTTP+SSE transport section.

Audit source: ai-mcp-developer audit, FINDING-017