chore: migrate to pnpm and update Node.js maintenance

[azu]

Summary

• Migrate from yarn v1 to pnpm
• Update Node.js versions from 12, 14 to 20, 22, 24 (LTS)
• Update GitHub Actions to latest versions
• Update repository settings and labels

Changes

• Package Manager Migration

  • Add packageManager field to package.json
  • Replace yarn.lock with pnpm-lock.yaml
  • Update workflow to use pnpm

• Node.js Version Updates

  • Update test matrix: Node.js 12, 14 → 20, 22, 24

• GitHub Actions Updates

  • actions/checkout: v2.7.0 → v5.0.0
  • actions/setup-node: v2.5.2 → v5.0.0

• Repository Configuration

  • Add/update GitHub labels (Type: Maintenance, Priority labels, etc.)
  • Add .github/release.yml
  • Add workflow permissions for security

Verification

• No uncommitted files
• pnpm installed and working
• No old Node.js versions in workflows
• All checks passing