uSync.Umbraco.Commerce — Umbraco 18 upgrade, repo standards, tax class fix + 17.1.0 release

Repo: Jumoo/uSync.Umbraco.Commerce

Upgraded to Umbraco 18 / Umbraco Commerce 18.1.0 on v18/main — pure package-version bump, no code changes needed
Added repository standards (CODE_OF_CONDUCT, SECURITY, CHANGELOG, CLAUDE.md, editorconfig, Directory.Build.props, GitVersion, locked packages.lock.json) and GitHub Actions CI/CD (PR build, package build, NuGet trusted-publishing release, CodeQL) via PR #11, merged
Ported the same repo standards to the v17/main line via PR #12, merged — 17.x-specific version numbers and lock file, code unchanged
Verified both PRs' dotnet-build.yml checks actually ran and passed in CI, not just locally
Fixed country/region tax rates not being restored on import — the serializer wrote them under a <TaxClasses> element but read them back looking for <TaxRates>, so import always saw an empty list and wiped existing overrides instead of restoring them (#10)
Fixed the tax rate value itself round-tripping as 0% — it was serialized via TaxRate.ToString() ("20.00%") instead of the underlying decimal, which failed to parse back on import
Fixed a related bug in ShippingMethodSerializer where a collection-diff compared the live country/region list against itself, so stale allowances never got removed on import
Released 17.1.0 to NuGet — first non-rc release of the 17.x line — with a matching GitHub release
Added umbraco-marketplace.json for the Umbraco Marketplace listing