chore: clean up OAuth env vars and add seal secret to dev
- Remove unused OAUTH_CLIENT_ID, OAUTH_REDIRECT_URI, OAUTH_PRIVATE_JWK from .env.prod.example
- Add OAUTH_SEAL_SECRET to .env.dev for local development
- Clarify that OAUTH_SEAL_SECRET is required, client secret/kid are optional
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>