docs: add proxy setup guide (ANTHROPIC_BASE_URL)

[BlueBoobyAI]

Summary

gstack skills work transparently through any OpenAI-compatible proxy that Claude Code can route through. This PR documents how:

• gstack skills make standard Claude API tool calls — they don't bypass the base URL mechanism
• ANTHROPIC_BASE_URL is a Claude Code native env var that routes ALL API traffic
• Zero gstack code changes needed. No skill modifications, no host config entries, no hook changes

The reference implementation is free-claude-code (Docker container, localhost:8082), which routes to DeepSeek V4 Flash at ~90% cost reduction versus Anthropic direct pricing.

What's included

New file: docs/howto-proxy-setup.md

• Architecture diagram showing the request flow
• Setup instructions (two env vars, no gstack config changes)
• Real cost comparison table (DeepSeek V4 Flash: 95% input savings, 96% output savings)
• Known patterns (team config, multi-provider routing, CI/CD, debugging)
• Limitations section (latency, provider availability, API feature parity)

Evidence

Verified gstack + proxy works in production. This Claude Code session has both gstack skills loaded AND ANTHROPIC_BASE_URL=http://localhost:8082 pointing at a free-claude-code proxy container. All 53 installed skills are discoverable and functional.

The proxy is healthy and routing requests. gstack skills make standard Claude API tool calls -- routing is handled by Claude Code, not by gstack. No gstack code paths needed modification.

Testing

Documentation-only PR -- no code changes. The pattern is tested by running gstack skills through the proxy (what happens right now in this session).

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