release: prepare v6.0.0

[davidgamero]

Summary

• Bump version to 6.0.0 (major: node20 → node24 runtime upgrade)
• Update CHANGELOG.md with all changes since v5.1.0
• Regenerate package-lock.json

Breaking Change

• Node.js runtime upgraded from node20 to node24 (Update Node.js runtime from node20 to node24 #504). Consumers using @v5 will need to update to @v6.

Changes since v5.1.0

• Update Node.js runtime from node20 to node24 #504 Update Node.js runtime from node20 to node24
• docs: update action version references in README to latest majors #500 Update action version references in README to latest majors
• Security dependency bumps (undici, vite, vitest, actions group)

Validation

• tsc --noEmit ✅
• npm run build ✅ (produces valid ESM bundle)
• npm test ✅ (254/254 tests pass)
• npm run format-check ✅
• package.json, package-lock.json, CHANGELOG.md versions all sync at 6.0.0
• action.yml uses node24 runtime

Merging triggers the release workflow via CHANGELOG push to main.