Add clean Protégé 6 plugin registry

[matthewhorridge]

Summary

• add a separate Protégé 6 plugin registry
• include only HermiT 1.5.0, DL Query 5.0.0, and OWLViz 6.0.0
• keep reviewed update descriptors alongside the registry
• validate every plugin download against its declared OSGi symbolic name and version
• run the validation in GitHub Actions when the registry changes
• document the policy for adding Protégé 6 plugins

Verification

The registry validator downloads all three Maven Central artifacts and confirms their bundle identities and versions.

This is the registry half of protegeproject/protege#1390 and should merge before the corresponding Protégé default-registry change.