fix: use nixos-version --json for configuration revision

- Use `nixos-version --json` command instead of reading files directly
- Add nixpkgs_rev and nixos_version labels to nixos_flake_info metric
- Show "unknown" for current_rev when system.configurationRevision not set
- Document configurationRevision setup in README

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

main.go

@@ -15,7 +15,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const version = "0.2.1"
+const version = "0.2.2"
 
 func main() {
 	cfg, err := config.Parse()