Document and audit family label taxonomy

[cmccomb]

Summary

Write down the approved label taxonomy for the design-research family and add a lightweight audit so unused legacy labels do not drift back in.

Why

• We just had to do a manual cleanup of stale labels across the family.
• Shared core labels are now standardized, but repo-specific taxonomies still need a clear source of truth.
• A small audit or checklist would make future repo setup much more reliable.

Suggested scope

• Document the shared core labels and the allowed repo-specific extension pattern.
• Note which generic/default GitHub labels should not be reintroduced.
• Add a repeatable audit step or script for checking label drift.
• Link to any repo-specific exceptions that are intentionally preserved.