feat: add ChatGPT sign-in for AI teacher

[DavidDing16]

Summary

• Add connection profiles and a provider registry while preserving the existing OpenAI-compatible API-key path.
• Add ChatGPT sign-in through the official Codex App Server, including account status, model discovery, vision input, streaming explanations, cancellation, and logout.
• Remove the legacy Python LLM review path so teacher review is routed through the active provider runtime.
• Add an always-visible AI讲这步 control beside the teacher composer. It creates a new current-move explanation only when clicked; navigating moves does not create requests.
• Update the README, contributor setup, privacy notes, and implementation documentation.

Verification

• TypeScript checks for the Node and renderer projects
• Targeted contract tests: provider login/runtime, teacher-agent runtime, and current-move CTA (11 passing tests)
• Manual UI check: CTA is always visible, click-triggered, and disabled while a teacher task is running
• Manual ChatGPT check: a ChatGPT Pro account successfully used gpt-5.6-luna for an AI teacher explanation
• Full cross-platform build and CI verification (GitHub Actions)

Privacy and compatibility

• OAuth credentials remain managed by the official Codex client/App Server; GoAgent does not read, copy, or store OAuth tokens.
• Existing OpenAI-compatible API-key settings are migrated into a connection profile and remain supported.
• When the user explicitly selects ChatGPT, the same required teaching evidence (board image, KataGo evidence, and selected knowledge excerpts) is sent through that provider.

Reviewer notes

• This PR intentionally changes the teacher integration boundary from a single protocol-specific implementation to a provider runtime.
• The separate design and security rationale is in docs/MULTI_PROVIDER_MODEL_ACCESS.md.

[DavidDing16]

@wimi321 这份 Draft PR 新增了 ChatGPT 登录 / Codex App Server provider，并补上了当前手的 AI 讲解入口。方便时烦请 review，尤其希望确认 provider runtime 边界、旧 API Key 的兼容迁移，以及 OAuth token 的隐私处理。谢谢！