ci: test on macOS and Windows, and actually run go test

[sundaram2021]

What

Expands the reusable test workflow (.github/workflows/test.yml) to run on
Linux, macOS, and Windows via an OS matrix, and adds a real go test ./...
step (the job was named "Go Test & Vet" but only ran go vet + a build check).

Resolves #307

Why

• We ship macOS and Windows binaries via GoReleaser but only tested on
  ubuntu-latest, so platform-specific regressions (keyring, paths, terminal
  handling) could ship unnoticed — see CLI is broken on windows git bash #80.
• The suite wasn't being executed at all, on any OS.

Changes

• Add matrix os: [ubuntu-latest, macos-latest, windows-latest] with
  fail-fast: false.
• Add a go test ./... step.
• Make the build step cross-platform: move CGO_ENABLED=0 to a job-level
  env: block (the inline VAR=value prefix is bash-only and breaks on
  Windows' default PowerShell), and replace go build -o /dev/null ./ with
  go build ./... (/dev/null doesn't exist on Windows).

Since test.yml is called by both main.yml and release.yml, this covers
PR CI and the release gate in one change.

Testing

• go test ./... passes locally — 30 tests across cmd, pkg/ai,
  pkg/config, pkg/util.
• GOOS=windows go vet ./... and GOOS=windows go build ./... are clean.
• CI on this PR now runs the suite on all three OSes.

[sundaram2021]

please review it and let me know if any changes needed