fix(version): don't add "v" prefix to non-semver VERSION strings

When VERSION is set to a non-tag value like "main" (from goreleaser or
ldflags), the init() function unconditionally prepended "v", producing
"vmain". Now only add the "v" prefix when doing so produces a valid
semver string, leaving branch names and other non-semver values as-is.

version/version.go

@@ -90,10 +90,13 @@ func init() {
 		VERSION = "dev-snapshot"
 	} else {
 		if !strings.HasPrefix(VERSION, "v") {
-			VERSION = "v" + VERSION
+			vVersion := "v" + VERSION
+			if semver.IsValid(vVersion) {
+				VERSION = vVersion
+			}
 		}
-		if !semver.IsValid(VERSION) {
-			slog.Default().Warn("invalid version number", "version", VERSION)
+		if !semver.IsValid(VERSION) && VERSION != "dev-snapshot" {
+			slog.Default().Info("non-semver version", "version", VERSION)
 		}
 	}
 	if bi, ok := debug.ReadBuildInfo(); ok {