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.test.ts

@@ -0,0 +1,20 @@
+import { describe, it, expect } from "vitest";
+import type { PlopTypes } from "@turbo/gen";
+import generator from "./config";
+
+describe("core-package generator", () => {
+  it("is registered with an empty choices list initially", () => {
+    const captured: Array<{ name: string; def: PlopTypes.PlopGeneratorConfig }> = [];
+    const plopMock = {
+      setHelper: () => {},
+      setGenerator: (name: string, def: PlopTypes.PlopGeneratorConfig) =>
+        captured.push({ name, def }),
+    } as unknown as PlopTypes.NodePlopAPI;
+    generator(plopMock);
+    const corePkg = captured.find((c) => c.name === "core-package");
+    expect(corePkg).toBeDefined();
+    const prompts = corePkg!.def.prompts as Array<{ name: string; choices: unknown[] }>;
+    expect(prompts[0]!.name).toBe("name");
+    expect(prompts[0]!.choices).toEqual([]);
+  });
+});