Add unit-test-vue-pinia skill (#1005)

skills/unit-test-vue-pinia/references/pinia-patterns.md

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+# Pinia Testing Snippets (Cookbook-Aligned)
+
+Use these patterns directly when writing tests with `@pinia/testing`.
+
+## Component mount with `createTestingPinia`
+
+```ts
+import { mount } from "@vue/test-utils";
+import { createTestingPinia } from "@pinia/testing";
+import { vi } from "vitest";
+
+const wrapper = mount(ComponentUnderTest, {
+  global: {
+    plugins: [
+      createTestingPinia({
+        createSpy: vi.fn,
+      }),
+    ],
+  },
+});
+```
+
+## Execute real actions
+
+Use this only when behavior inside the action must run.
+If the test only checks call/no-call expectations, keep default stubbing (`stubActions: true`).
+
+```ts
+const wrapper = mount(ComponentUnderTest, {
+  global: {
+    plugins: [
+      createTestingPinia({
+        createSpy: vi.fn,
+        stubActions: false,
+      }),
+    ],
+  },
+});
+```
+
+## Seed starting state
+
+```ts
+const wrapper = mount(ComponentUnderTest, {
+  global: {
+    plugins: [
+      createTestingPinia({
+        createSpy: vi.fn,
+        initialState: {
+          counter: { n: 10 },
+          profile: { name: "Sherlock Holmes" },
+        },
+      }),
+    ],
+  },
+});
+```
+
+## Use store in test and assert action call
+
+```ts
+const pinia = createTestingPinia({ createSpy: vi.fn });
+const store = useCounterStore(pinia);
+
+store.increment();
+expect(store.increment).toHaveBeenCalledTimes(1);
+```
+
+## Add plugin under test
+
+```ts
+const wrapper = mount(ComponentUnderTest, {
+  global: {
+    plugins: [
+      createTestingPinia({
+        createSpy: vi.fn,
+        plugins: [myPiniaPlugin],
+      }),
+    ],
+  },
+});
+```
+
+## Override and reset getters for edge tests
+
+```ts
+const pinia = createTestingPinia({ createSpy: vi.fn });
+const store = useCounterStore(pinia);
+
+store.double = 42;
+expect(store.double).toBe(42);
+
+// @ts-expect-error test-only reset
+store.double = undefined;
+```