ci: add tag-based release workflow and npm publish scripts

[posokhin]

Summary

• Adds release:patch, release:minor, release:major scripts to package.json — each bumps the version, creates a git tag, and pushes with --follow-tags
• Adds .github/workflows/release.yml triggered on v*.*.* tags: runs full CI (type-check, lint, test, build), publishes to npm using NPM_TOKEN secret, and creates a GitHub Release with auto-generated notes via softprops/action-gh-release@v2
• Exports Layer4Error from the public entry point (src/index.ts)

Release flow

npm run release:patch   # bumps 1.0.0 → 1.0.1, creates tag v1.0.1
  └─ git push --follow-tags
        └─ GitHub Actions: tag v*.*.*
              ├─ type-check · lint · test · build
              ├─ npm publish
              └─ GitHub Release (auto notes)

Prerequisites (one-time manual step)

Add NPM_TOKEN secret in GitHub:

Settings → Secrets and variables → Actions → New repository secret
  Name:  NPM_TOKEN
  Value: <npm automation token>

Test plan

• Merge this PR
• Add NPM_TOKEN secret in repo settings
• Run npm publish --dry-run locally to verify package contents
• Run npm run release:patch and check the Actions tab
• Verify package appears on npmjs.com and release appears on GitHub Releases

🤖 Generated with Claude Code