openai/gpt-chat-latest → B (3:1)

Side B corrects the rank-history thread index semantics by making `thread_post_index` consistently 0-based, replacing silent `unwrap_or(0)` fallbacks with `expect(...)` to catch invariant violations, updating UI links, documentation, and adding integration tests that verify the behavior. Side A is a targeted but much smaller fix that defines `GITHUB_API_BASE_URL` to prevent a `NameError` in the GitHub OAuth callback when the API base URL is otherwise missing.