refactor(ai): make metric bucket ordering deterministic

Co-Authored-By: Virgil <virgil@lethean.io>

ai/metrics.go

@@ -170,7 +170,8 @@ func Summary(events []Event) map[string]any {
 	}
 }
 
-// sortedCountPairs returns a slice of key-count pairs sorted by count descending.
+// sortedCountPairs returns a slice of key-count pairs sorted by count descending,
+// with key ascending as a deterministic tie-breaker.
 func sortedCountPairs(counts map[string]int) []map[string]any {
 	type entry struct {
 		key   string
@@ -182,7 +183,10 @@ func sortedCountPairs(counts map[string]int) []map[string]any {
 	}
 
 	slices.SortFunc(entries, func(a, b entry) int {
-		return cmp.Compare(b.count, a.count)
+		if result := cmp.Compare(b.count, a.count); result != 0 {
+			return result
+		}
+		return cmp.Compare(a.key, b.key)
 	})
 
 	result := make([]map[string]any, len(entries))

ai/metrics_test.go

@@ -309,3 +309,20 @@ func TestSortedCountPairs_Good_Ordering(t *testing.T) {
 		t.Errorf("expected third key 'a', got %v", result[2]["key"])
 	}
 }
+
+func TestSortedCountPairs_Good_TieBreakByKey(t *testing.T) {
+	m := map[string]int{"beta": 2, "alpha": 2, "gamma": 1}
+	result := sortedCountPairs(m)
+	if len(result) != 3 {
+		t.Fatalf("expected 3 entries, got %d", len(result))
+	}
+	if result[0]["key"] != "alpha" {
+		t.Errorf("expected tie-break to prefer alpha, got %v", result[0]["key"])
+	}
+	if result[1]["key"] != "beta" {
+		t.Errorf("expected beta second, got %v", result[1]["key"])
+	}
+	if result[2]["key"] != "gamma" {
+		t.Errorf("expected gamma last, got %v", result[2]["key"])
+	}
+}