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>

collector/flake_test.go

@@ -2,23 +2,46 @@ package collector
 
 import (
 	"encoding/json"
-	"os"
+	"os/exec"
 	"testing"
 )
 
-func TestGetCurrentSystemRevision(t *testing.T) {
-	// Skip if not on NixOS with system.configurationRevision set
-	if _, err := os.Stat(configRevisionPath); os.IsNotExist(err) {
-		t.Skip("not running on NixOS with system.configurationRevision set")
+func TestGetNixosVersionInfo(t *testing.T) {
+	// Skip if nixos-version command is not available
+	if _, err := exec.LookPath("nixos-version"); err != nil {
+		t.Skip("nixos-version command not available")
 	}
 
-	rev, err := getCurrentSystemRevision()
+	info, err := getNixosVersionInfo()
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	// Just check it returns something reasonable
-	t.Logf("current system revision: %s", rev)
+	t.Logf("NixOS version info: configurationRevision=%s, nixosVersion=%s, nixpkgsRevision=%s",
+		info.ConfigurationRevision, info.NixosVersion, info.NixpkgsRevision)
+}
+
+func TestNixosVersionInfoUnmarshal(t *testing.T) {
+	jsonData := `{
+		"configurationRevision": "138aeb9e450890efd7ca3be6159b1eb22256c2e3",
+		"nixosVersion": "25.11.20260203.e576e3c",
+		"nixpkgsRevision": "e576e3c9cf9bad747afcddd9e34f51d18c855b4e"
+	}`
+
+	var info nixosVersionInfo
+	if err := json.Unmarshal([]byte(jsonData), &info); err != nil {
+		t.Fatal(err)
+	}
+
+	if info.ConfigurationRevision != "138aeb9e450890efd7ca3be6159b1eb22256c2e3" {
+		t.Errorf("expected configurationRevision 138aeb9e450890efd7ca3be6159b1eb22256c2e3, got %s", info.ConfigurationRevision)
+	}
+	if info.NixosVersion != "25.11.20260203.e576e3c" {
+		t.Errorf("expected nixosVersion 25.11.20260203.e576e3c, got %s", info.NixosVersion)
+	}
+	if info.NixpkgsRevision != "e576e3c9cf9bad747afcddd9e34f51d18c855b4e" {
+		t.Errorf("expected nixpkgsRevision e576e3c9cf9bad747afcddd9e34f51d18c855b4e, got %s", info.NixpkgsRevision)
+	}
 }
 
 func TestFlakeLocksUnmarshal(t *testing.T) {