Add unit-test-vue-pinia skill

[rbgmulmb]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, or workflow file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

This PR adds a new unit-test-vue-pinia skill for Vue 3 + TypeScript + Vitest + Pinia unit testing.

The goal is to correct a recurring model failure mode: models tend to hand-roll or over-mock Pinia stores in component tests, even though Pinia already provides a built-in testing utility via @pinia/testing and createTestingPinia. This skill teaches the model to prefer Pinia's supported testing patterns, choose the lightest valid setup, and keep assertions behavior-first instead of implementation-coupled.

The skill includes:

• guidance for when to use createTestingPinia vs createPinia
• accepted minimal Pinia setups so valid tests are not incorrectly rewritten
• examples for action stubbing, real action execution, state seeding, plugin usage, and getter overrides
• a checked-in local reference file with cookbook-aligned Pinia testing snippets

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.