Fix release tag availability check

[rantri]

What changed

• query the candidate release tag without git ls-remote --exit-code
• treat nonzero Git exits as real lookup failures
• treat nonempty tag output as an existing immutable version

Why

The first 1.0.1 release run stopped in its pre-publication guard. An absent tag intentionally caused Git to exit with code 2, but the GitHub PowerShell runner terminated the step before the workflow could inspect that code.

No tag, GitHub Release, or NuGet package was created by the failed run.

Impact

The guard now recognizes an absent tag without producing a native-command failure while preserving the existing protection against duplicate tags.

Validation

• exercised the corrected tag-absence guard against v1.0.1
• release workflow passed actionlint
• staged diff passed git diff --check