✨ Add built-in GitHub Models provider

[a5c-agent]

Summary

Adds a built-in github model provider to codex-rs and updates docs with configuration and usage instructions.

• Provider id: github
• Name: "GitHub Models"
• Base URL: https://models.github.ai/inference
• Auth: GITHUB_TOKEN (Bearer)
• Wire API: chat (OpenAI-compatible)

Rationale

GitHub Models exposes an OpenAI-compatible API hosted on Azure AI Inference. This provider enables users to target GitHub-hosted models by setting model_provider = "github" and providing GITHUB_TOKEN.

Changes

• codex-rs/core/src/model_provider_info.rs: add github entry to built_in_model_providers()
• docs/config.md: document provider usage and config snippet

No other files are changed; specifically excludes any .a5c and .github automation files.

Testing

• Ran cargo fmt and cargo clippy --fix -p codex-core --all-features --tests.
• Ran cargo test -p codex-core and cargo test --all-features with network disabled; all tests passed.

Usage

Example config:

# Use GitHub Models
model_provider = "github"
model = "openai/gpt-5"  # or any supported model

[model_providers.github]
name = "GitHub Models"            # built-in
base_url = "https://models.github.ai/inference"
env_key = "GITHUB_TOKEN"          # Bearer token for GitHub Models
wire_api = "chat"

Ensure GITHUB_TOKEN has access to GitHub Models. See the GitHub docs for details.

Notes

• This change follows the existing provider pattern; no behavior changes outside the new provider.
• Happy to adjust naming, descriptions, or docs as preferred.

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[tmuskal]

I have read the CLA Document and I hereby sign the CLA

[tmuskal]

fixes #2069

[a5c-agent]

I have read the CLA Document and I hereby sign the CLA