- Bump _config_version 8 → 9
- Move stale ANTHROPIC_TOKEN clearing into 'if current_ver < 9' block
so it only runs once during the upgrade, not on every migrate_config()
- ANTHROPIC_TOKEN is still a valid auth path (OAuth flow), so we don't
want to clear it repeatedly — only during the one-time migration from
old setups that left it stale
- Add test_skips_on_version_9_or_later to verify one-time behavior
- All tests set config version 8 to trigger migration