Hookpack manifest mirrors CustomInstrumentDef: rich features,
TypeScript entry, optional file-based icon. Folder name is the
pack id.
HookPackConfig holds FeatureState per feature, producing the
same agent-JSON shape as custom instruments. Hookpack sources
compile through the shared TypeScript pipeline.
Import sits next to "New Custom Instrument..."; Export on the
custom-instrument context menu. Both UIs.
