chore(plugins): gate registry script in CI, drop completed migration script

[datlechin]

What

Cleanup of the two stray Python files found in a repo audit:

• Wire test_update_registry.py into CI. The test existed but nothing ran it. It now runs once in the resolve-tags job (before any plugin builds), so the registry prune logic (drops null pluginKitVersion binaries, keeps the two newest) is guarded on every plugin release/dispatch.
• Remove scripts/migrate-registry-v1-to-v2.py. Its own docstring marks it a one-time v1-to-v2 migration. The live registry is already schemaVersion: 2, so the migration is done and the script is dead.

Not touched

update-registry.py stays (active, used by build-plugin.yml).

No app code, no user-facing strings.