ellmercodex - 'ChatGPT' Plan Provider for 'ellmer'

Connects 'ellmer' chats to a 'ChatGPT' subscription through 'OpenAI''s documented "Sign in with ChatGPT" flow for open-source, locally hosted apps <https://developers.openai.com/siwc/token-sharing-open-source>. Provides a stable, explicitly bounded core of the public 'ellmer' (>= 0.5.0) Chat object, including browser sign-in with dynamic client registration, package-scoped credential storage, redacted diagnostics, multi-turn text, streaming, structured-output, function-tool, asynchronous, cancellation, cloning, metadata, multimodal chat, account-specific model discovery, and reasoning-effort selection through the public 'Responses' API. Parallel and batch helper functions are explicitly unsupported because 'ChatGPT' plan usage is stream-only. This independent third-party integration is not endorsed by 'OpenAI'.