Add reusable workflow for Node.js and pnpm setup

[Copilot]

This PR consolidates the repetitive Node.js and pnpm installation steps that were duplicated across multiple GitHub Actions workflows by creating a reusable composite action.

Changes Made

Created Reusable Composite Action

• Location: .github/actions/setup-node-pnpm/
• Features:
  • Configurable Node.js version (defaults to LTS)
  • Automatic pnpm setup using project's configured version
  • Optional pnpm store caching for CI performance
  • Configurable lockfile handling (frozen vs regular)
  • Proper GITHUB_TOKEN environment variable handling

Updated Workflows

All three existing workflows now use the new composite action:

• ci.yml: Uses caching and frozen lockfile for strict CI requirements
• deploy.yml: Uses flexible settings appropriate for deployment
• copilot-setup-steps.yml: Uses flexible settings for development environment

Benefits

• Reduced code duplication: Eliminated ~40 lines of repeated setup code across workflows
• Improved maintainability: Single source of truth for Node.js/pnpm setup configuration
• Flexible configuration: Different workflows can specify appropriate settings via inputs
• Better documentation: Clear usage examples and comprehensive input descriptions

Usage Example

- name: Setup Node.js and pnpm
  uses: ./.github/actions/setup-node-pnpm
  with:
    enable-cache: true
    frozen-lockfile: true

The action automatically handles Node.js LTS installation, pnpm setup, optional caching, and dependency installation while maintaining full compatibility with existing workflow requirements.

Fixes #30.

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