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>

turbo/generators/package.json

@@ -2,5 +2,11 @@
   "name": "@repo/turbo-generators",
   "private": true,
   "version": "0.0.0",
-  "type": "module"
+  "type": "module",
+  "scripts": {
+    "test": "vitest run"
+  },
+  "devDependencies": {
+    "vitest": "^3.1.0"
+  }
 }