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/config.ts

@@ -491,6 +491,38 @@ export default function generator(plop: PlopTypes.NodePlopAPI): void {
     },
   });
 
+  /**
+   * Turbo generator: `core-package`
+   *
+   * Scaffolds an optional core package back into a slimmed template. Each
+   * name maps to a verbatim snapshot of the package captured at generator-add
+   * time. Phases 3-6 each register one entry in CORE_PACKAGE_GENERATORS.
+   */
+  const CORE_PACKAGE_GENERATORS: Record<string, () => PlopTypes.ActionType[]> =
+    {
+      // Phases 3-6 each insert one entry here.
+    };
+
+  plop.setGenerator("core-package", {
+    description: "Scaffold an optional core package (realtime, events, trpc, ui)",
+    prompts: [
+      {
+        type: "list",
+        name: "name",
+        message: "Which optional core package?",
+        choices: [],
+      },
+    ],
+    actions: (answers) => {
+      const a = answers as { name: string };
+      const handler = CORE_PACKAGE_GENERATORS[a.name];
+      if (!handler) {
+        throw new Error(`No generator for core-package '${a.name}'`);
+      }
+      return handler();
+    },
+  });
+
   /**
    * Turbo generator: `job`
    *