Implement publish script: sync gonka-proxy-service → gonka-proxy (public)

[mingles-agent]

Context

gonka-proxy-service (private) is the single source of truth for all development. gonka-proxy (public) receives only selected changes — published manually via a script.

Task

Create scripts/publish-public.sh in gonka-proxy-service that:

1. Verifies we're on main with a clean working tree
2. Strips MinglesAI-specific content from README (e.g. gonka-gateway.mingles.ai URL)
3. Pushes sanitized code to upstream/main (i.e. MinglesAI/gonka-proxy)

The upstream remote is already configured in the repo pointing to MinglesAI/gonka-proxy.git.

Acceptance criteria

• Script runs without errors from repo root
• gonka-proxy public repo gets updated without any MinglesAI-internal URLs
• README in public repo is clean and usable standalone