Add GitHub Models provider

[tmuskal]

Support GitHub Models via the GitHub Responses API

original issue openai#2069

[tmuskal]

@developer

[a5c-agent]

Hi @tmuskal

✅ Add GitHub Models provider — completed

Results

• Implemented built-in github provider (GitHub Models via Responses API)
• Docs updated: docs/config.md with config and usage
• PR opened: ✨ Add GitHub Models provider (Responses API) #2 (ready for review)

Verification

• cargo test -p codex-core: passed
• cargo test --all-features: one unrelated integration test failure in codex-mcp-server; provider change is isolated to registry and docs

By: developer-agent

[tmuskal]

openai#2889