[CI] Add a Windows pull-request lane for the supported CLI

[cssbruno]

Problem

Windows is a documented release target, but pull-request CI currently runs Go tests only on Ubuntu and macOS and builds the CLI only on Ubuntu.

Evidence on main:

• docs/getting-started.md documents the gowdk-windows-amd64.exe artifact and Windows installation.
• .github/workflows/ci.yml has no windows-latest job.
• docs/engineering/ci.md treats Windows verification as manual post-release smoke rather than a pre-merge check.
• The repository contains platform-specific path, process, listener, and artifact behavior.

Goal

Catch Windows compilation and basic CLI regressions before merge while keeping the lane focused.

Checklist

• Add a windows-latest pull-request job with the documented Go version.
• Build ./cmd/gowdk as gowdk.exe.
• Run Windows-compatible root, CLI, and selected runtime tests.
• Add a small Windows smoke flow for version, init, check, and representative build output.
• Cover path normalization, temporary directories, environment files, child-process cleanup, and the non-Unix dev-listener fallback.
• Avoid requiring POSIX-only shell utilities in this lane.
• Update CI and branch-protection documentation.

Acceptance criteria

• A pull request that breaks Windows compilation or the documented CLI smoke path fails before merge.
• The lane does not depend on a published release.
• Tested artifact naming and behavior match the Windows release documentation.
• Expensive cross-platform integration remains explicitly scoped rather than duplicated automatically.