chore: add copilot instructions and agent setup steps workflow

.github/copilot-instructions.md

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+# GitHub Action: Text Variables
+
+This is a JavaScript-based GitHub Action that replaces mustache-style variables
+(`{{ variable }}`) in text templates. The action is designed to enable dynamic
+content creation for GitHub workflows, particularly for customizing Issue/PR
+descriptions, comments, and other text content.
+
+## Code Standards
+
+### Required Before Each Commit
+
+- Run `npm run format:write` to format code with Prettier
+- Run `npm run lint` to check code with ESLint
+- Run `npm run test` to execute all tests
+- Run `npm run all` to run the complete validation pipeline (format, lint, test,
+  coverage, package)
+
+## Repository Structure
+
+- `src/`: Core JavaScript source code
+  - `index.js`: Entry point for the action
+  - `main.js`: Main logic for template processing and variable replacement
+- `__tests__/`: Jest test files and sample templates
+- `action.yml`: GitHub Action metadata and input/output definitions
+- `dist/`: Compiled/bundled output (generated, do not edit directly)
+
+## Code Quality
+
+1. Follow JavaScript best practices and Node.js conventions
+1. Use async/await for asynchronous operations
+1. Write comprehensive Jest tests for unit testing.
+1. Write integration tests for end-to-end validation of the GitHub Action using
+   workflows in `.github/workflows/ci.yml`
+1. Use descriptive variable names and add comments for complex logic

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

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+name: 'Copilot Setup Steps'
+
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+permissions:
+  contents: read
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: .node-version
+          cache: 'npm'
+
+      - name: Install JavaScript dependencies
+        run: npm ci