chore: add copilot agent configs (vitejs#20422)

Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

Author: sapphi-red

.github/copilot-instructions.md

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+This is a TypeScript project that implements a frontend build tooling called Vite. Please follow these guidelines when contributing:
+
+## Code Standards
+
+### Required Before Each Commit
+
+- Run `pnpm run lint` to ensure that your code adheres to the code standards.
+- Run `pnpm run format` to format your code.
+
+### Development Flow
+
+- Build: `pnpm run build`
+- Test: `pnpm run test` (uses Vitest and Playwright)
+
+## Repository Structure
+
+- `docs/`: Documentation.
+- `packages/create-vite`: Contains the source code for the `create-vite` command.
+- `packages/plugin-legacy`: Contains the source code for `@vitejs/plugin-legacy`.
+- `packages/vite`: Contains the source code for the Vite core.
+- `playground/`: E2E tests
+
+## Key Guidelines
+
+1. Follow TypeScript best practices.
+2. Maintain existing code structure and organization.
+3. Write tests for new functionality. Prefer unit tests if it can be tested without using mocks. E2E tests should be added in the `playground/` directory.
+4. Never write comments that explain what the code does. Instead, write comments that explain why the code does what it does.
+5. Suggest changes to the documentation if public API changes are made.

.github/workflows/copilot-setup-steps.yml

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+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@a7487c7e89a18df4991f7f222e4898a00d66ddda # v4.1.0
+
+      - name: Set node version to 22
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: "pnpm"
+
+      - name: Install deps
+        run: pnpm install
+
+      # Install playwright's binary under custom directory to cache
+      - name: (non-windows) Set Playwright path and Get playwright version
+        if: runner.os != 'Windows'
+        run: |
+          echo "PLAYWRIGHT_BROWSERS_PATH=$HOME/.cache/playwright-bin" >> $GITHUB_ENV
+          PLAYWRIGHT_VERSION="$(pnpm ls --depth 0 --json -w playwright-chromium | jq --raw-output '.[0].devDependencies["playwright-chromium"].version')"
+          echo "PLAYWRIGHT_VERSION=$PLAYWRIGHT_VERSION" >> $GITHUB_ENV
+      - name: (windows) Set Playwright path and Get playwright version
+        if: runner.os == 'Windows'
+        run: |
+          echo "PLAYWRIGHT_BROWSERS_PATH=$HOME\.cache\playwright-bin" >> $env:GITHUB_ENV
+          $env:PLAYWRIGHT_VERSION="$(pnpm ls --depth 0 --json -w playwright-chromium | jq --raw-output '.[0].devDependencies["playwright-chromium"].version')"
+          echo "PLAYWRIGHT_VERSION=$env:PLAYWRIGHT_VERSION" >> $env:GITHUB_ENV
+
+      - name: Install Playwright
+        # does not need to explicitly set chromium after https://github.com/microsoft/playwright/issues/14862 is solved
+        run: pnpm playwright install chromium