fix(semver): skip non-semver tags when picking latest existing tag

Rolling tags like 'v1' (and any other tag that doesn't parse as x.y.z
after prefix stripping) used to enter the latest-tag candidate set in
CalculateSemver. When such a tag shared a commit with a real semver tag
(e.g. v1 and v1.16.3 both pointing at the same release commit), go-git's
alphabetical tag iteration made 'v1' win, ParseExistingSemver bailed out
because '1' has only one component, and the calculator silently reset
the baseline to 0.0.0 — producing nonsense like 0.0.5 on this branch.

ListExistingTags now filters tags through a new IsParseableSemverTag
helper before recording them, so non-semver tags never participate in
latest-tag selection. The behavior change is invisible for repos that
only use proper vX.Y.Z tags, and it's covered by a new test table.

cmd/utils/version.go

@@ -83,6 +83,29 @@ func StripTagPrefix(tagName string, prefixes []string) string {
 	return result
 }
 
+// IsParseableSemverTag reports whether tagName looks like a proper semver tag
+// (X.Y.Z, optionally vX.Y.Z, optionally with -rc.N suffix) after configured
+// prefixes are stripped. Rolling tags like "v1" or "latest" return false so the
+// calculator can skip them when picking the latest existing tag — preventing a
+// rolling tag tied to the same commit as a real semver tag from "winning" the
+// alphabetical iteration in go-git and resetting the baseline to 0.0.0.
+func IsParseableSemverTag(tagName string, prefixes []string) bool {
+	clean := StripTagPrefix(tagName, prefixes)
+	if idx := strings.Index(clean, "-rc."); idx != -1 {
+		clean = clean[:idx]
+	}
+	parts := strings.Split(clean, ".")
+	if len(parts) < 3 {
+		return false
+	}
+	for _, p := range parts[:3] {
+		if len(extractNumber.FindAllString(p, -1)) == 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // ParseExistingSemver parses a semantic version from a tag name
 func ParseExistingSemver(tagName string, currentSemver SemVer, prefixes []string) SemVer {
 	Debug("Parsing existing semver", map[string]interface{}{"tag": tagName})