Guard release script against mismatched tags

[robmaceachern]

Summary

• Add an explicit existing-tag guard before gh release create runs.
• Fail when the requested version tag already exists at a different commit than the resolved release target.
• Allow the script to continue when an existing tag resolves to the same target commit.

Feedback Addressed

Addresses the concern from #622 (comment). The review was valid: gh release create --target only affects auto-created tags, so an existing tag without a release could have caused the release to be created from a different commit than the script printed.

Validation

• bash -n Scripts/release.sh
• Scripts/release.sh --version 9.9.9 --dry-run --no-open
• Scripts/release.sh --version 6.7.2 --target origin/main --dry-run --no-open fails with the new mismatched-tag error
• Scripts/release.sh --version 6.7.2 --target 6.7.2 --dry-run --no-open reaches the existing-release guard
• git diff --check

shellcheck is not installed locally.

[johnnewman-square]

This looks good. Thanks for making this tweak!