feat: add netssh version and --version

`netssh --version` used to be passed to ssh and failed with
"unknown option -- -". Release builds now embed the tag via
-X main.version; other builds fall back to the module build info.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/netssh/version.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"fmt"
+	"runtime/debug"
+
+	"github.com/spf13/cobra"
+)
+
+// version is set at build time: -ldflags "-X main.version=v1.2.3".
+var version = ""
+
+// buildVersion falls back to the module version for `go install` builds.
+func buildVersion() string {
+	if version != "" {
+		return version
+	}
+	if info, ok := debug.ReadBuildInfo(); ok && info.Main.Version != "" && info.Main.Version != "(devel)" {
+		return info.Main.Version
+	}
+	return "dev"
+}
+
+func versionCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "version",
+		Short: "Print the netssh version",
+		Args:  cobra.NoArgs,
+		Run: func(cmd *cobra.Command, args []string) {
+			fmt.Fprintf(cmd.OutOrStdout(), "netssh %s\n", buildVersion())
+		},
+	}
+}