Set up Copilot coding agent instructions

[Copilot]

Adds .github/copilot-instructions.md to configure GitHub Copilot coding agent behavior for this repository, following the best practices guide.

What's included

• Repo structure — monorepo layout, key directories, and what to ignore
• Build & test commands — yarn task vs NX equivalents, required flags (-c production), and approximate runtimes
• Commands to avoid — yarn task dev / yarn start (run indefinitely)
• Sandbox environments — location change (../storybook-sandboxes/), available templates, and fallback workflow when sandbox generation fails
• Development workflows — per-change-type guidance (code, UI, addon/framework)
• Contributing guidelines — logging conventions (node-logger vs client-logger), test coverage expectations, code quality checks (Prettier + ESLint)

Also fixes the Yarn version in the instructions (4.9.1 → 4.10.3) to match the actual packageManager field in package.json.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #33886

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[github-actions]

	Fails
🚫	PR is not labeled with one of: ["ci:normal","ci:merged","ci:daily","ci:docs"]
🚫	PR title must be in the format of "Area: Summary", With both Area and Summary starting with a capital letter Good examples: - "Docs: Describe Canvas Doc Block" - "Svelte: Support Svelte v4" Bad examples: - "add new api docs" - "fix: Svelte 4 support" - "Vue: improve docs"
🚫	PR description is missing the mandatory "#### Manual testing" section. Please add it so that reviewers know how to manually test your changes.

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