feat(sub2api): add Agent Identity import mode

[foreverninght]

Summary

• add a SUB2API import mode selector for OAuth vs Agent Identity
• switch the OpenAI Plus SUB2API tail flow to generate and import Agent Identity when selected
• reuse existing SUB2API group, proxy, and priority settings during Agent Identity import
• retry ChatGPT session/accessToken reads with a short initial delay, including the case where session exists but accessToken is not ready yet

Tests

• npm test