ci: no Windows job, so Windows regressions are never caught

[peteski22]

No CI job runs on Windows, so Windows regressions are invisible.

Every workflow is ubuntu-latest:

tests.yaml, golangci-lint.yaml, docs.yaml, deploy.yaml,
release.yaml, validate-registry.yaml, validate-goreleaser.yaml, validate-licenses.yaml

.goreleaser.yaml also builds linux and darwin only. Meanwhile #299 added internal/plugin/process_windows.go and internal/files/paths_windows_test.go already exists — we have Windows-specific code with zero automated coverage. Both Windows problems found while reviewing #299 were found by hand on a local Windows machine: the test-file breakage raised in that PR's review, and #311, which has been broken on main for some time.

Fix:

• add windows-latest to the tests.yaml job matrix
• decide whether known-failing Windows tests are skipped with a tracking issue or fixed first (files: executable discovery uses the Unix execute bit, so no plugin is discovered on Windows #311 blocks a green run today), so the job can go green and stay meaningful

Worth settling before more Windows-specific code lands, otherwise //go:build windows files are only ever compiled by whoever happens to run them locally.