Switch module list source of truth from bundled
external-official-modules.yaml to the remote marketplace registry
(registry/official.yaml) fetched via raw.githubusercontent.com.
- Rewrite ExternalModuleManager to fetch from GitHub with local fallback
- Simplify selectAllModules/getDefaultModules to use registry as single source
- Registry order controls display order; built_in flag prevents cloning
- Rename fallback file to registry-fallback.yaml in modules/
- Only show legacy migration message when legacy dirs actually exist
Brian Madison