ci(release): exclude current tag from compare-link base (review)

Addresses CodeRabbit review on #31: if a v$version tag already exists (e.g. a
re-run, or a tag pushed without a release), the previous-tag selection could pick
it and produce a self-referential Full Changelog compare link. Filter out
"v$version" before selecting the most recent tag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tablackburn

.github/workflows/PublishModuleToPowerShellGallery.yaml

@@ -133,7 +133,9 @@ jobs:
               # Append a compare link against the most recent existing tag. The v$version
               # tag does not exist yet (this step creates it), so the latest tag is the
               # previous release.
-              $previousTag = git tag --list 'v*' --sort=-version:refname | Select-Object -First 1
+              $previousTag = git tag --list 'v*' --sort=-version:refname |
+                  Where-Object { $_ -ne "v$version" } |
+                  Select-Object -First 1
               if ($previousTag) {
                   $body += "`n`n**Full Changelog**: https://github.com/$env:REPOSITORY/compare/$previousTag...v$version"
               }