Rename oauth command to login and use vit.json creds
- rename src/cmd/oauth.js to src/cmd/login.js and update CLI registration
- migrate credential storage from src/lib/env.js to src/lib/config.js
- switch credential keys to did/access_token/refresh_token/expires_at
- update pds-record, plc-verify, and firehose to read did from config
- add created_at and updated_at epoch timestamp stamping in saveConfig()
- remove src/lib/env.js and update README, CLAUDE, skills, and VOCAB docs