fix(installer): descriptive error when module definition missing after clone (#2377)

* fix(installer): throw descriptive error when module definition missing after clone

When a stable tag predates a module restructure (e.g. baut v1.14.0 had
payload/source dirs, but the registry pointed to skills/module.yaml which
only exists on main), findExternalModuleSource silently returned the
configured but non-existent path. This caused a confusing ENOENT inside
getFileList/copyModuleWithFiltering rather than a clear error.

Now throws with the version that was cloned and a --next hint when the
install channel was stable, so users know exactly how to recover.

Closes #2372

* style: fix prettier formatting in external-manager.js

* style: apply prettier formatting