Skill: HTTP/2 & SSE proxy configuration (Caddy/nginx)

## Context

A common developer pain point is "my shapes are slow in local dev" — caused by HTTP/1.1's 6-connection browser limit. The fix is non-obvious: set up an HTTP/2 reverse proxy. Similarly, SSE requires proxy buffering to be disabled.

## Scope

Create an `electric-proxy-config` skill covering:

1. **Problem explanation** — Why HTTP/1.1 causes slow shapes, how to diagnose
2. **Caddy setup** — Caddyfile for HTTP/2 + `flush_interval -1` for SSE
3. **nginx setup** — `proxy_buffering off`, HTTP/2 listener config
4. **Apache setup** — `flushpackets=on` for SSE
5. **Vercel/Cloudflare/AWS** — CDN-specific SSE and caching considerations
6. **Diagnosis** — How to tell if you're hitting the connection limit vs other issues

## Source

Identified during domain discovery skill test run. Kept as standalone skill since "shapes are slow" is a concrete troubleshooting moment distinct from general deployment.