Configurable registry list persisted to config/registries.json.
Image pulls try all registries in priority order — if primary fails,
fallback registries are attempted automatically. RPC endpoints:
registry.list, registry.add, registry.remove, registry.test.
Replaces hardcoded fallback logic with extensible registry system.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
