feat: add nixos_flake_info metric with current and remote revisions

Add a new info metric that exposes the current system's flake revision
and the latest remote revision as labels. This makes it easier to see
exactly which revision is deployed vs available.

Also adds version constant to Go code and extracts it in flake.nix,
providing a single source of truth for the version.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main.go

@@ -15,6 +15,8 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+const version = "0.2.0"
+
 func main() {
 	cfg, err := config.Parse()
 	if err != nil {
@@ -59,7 +61,7 @@ func main() {
 	defer stop()
 
 	go func() {
-		slog.Info("Starting server", "addr", cfg.ListenAddr)
+		slog.Info("Starting server", "version", version, "addr", cfg.ListenAddr)
 		if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
 			slog.Error("Server error", "error", err)
 			os.Exit(1)