Use shipped catalog-index metadata for upgrade assessments

[karthikjeeyar]

What does this change do?

Updates rhdh-upgrade-helper to use the shipped RHDH plugin catalog-index image as the authoritative product-data source, with the overlay repository as a fallback.

The upgrade assessment now:

• Resolves plugin artifacts from Package entity spec.dynamicArtifact
• Distinguishes bundled, OCI, and NPM artifact transitions
• Supports package aliases and renamed package scopes
• Computes {{inherit}} references for eligible GA OCI plugins
• Records product-data source and fidelity
• Parses target release versions and default plugin state
• Adds safe extraction and parsing scripts
• Updates migration guidance, scoring, and architecture documentation

Testing

• pytest — 18 passed
• ruff check . — passed
• ruff format --check . — passed
• git diff --check — passed
• bash -n skills/rhdh-upgrade-helper/scripts/extract-catalog-index.sh — passed

Related issue

Notes

The catalog-index extractor prefers:

1. oc image extract
2. Podman
3. Docker
4. Overlay repository fallback