feat(generators): add core-package entry + dispatch table (empty)

Registers pnpm turbo gen core-package with an empty choices list and a
dispatch table that throws for unknown names. Adds vitest config + test
script to turbo/generators and adds it to the pnpm workspace so tests
run via pnpm --filter @repo/turbo-generators test.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pnpm-workspace.yaml

@@ -1,3 +1,4 @@
 packages:
   - "apps/*"
   - "packages/*"
+  - "turbo/generators"