feat: dynamic container registry with fallback

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>

core/archipelago/src/container/mod.rs

@@ -2,6 +2,7 @@ pub mod data_manager;
 pub mod dev_orchestrator;
 pub mod docker_packages;
 pub mod image_versions;
+pub mod registry;
 
 pub use dev_orchestrator::DevContainerOrchestrator;
 pub use docker_packages::DockerPackageScanner;