Clarify run-if skip reasons with the resolved variable value.

Failed conditions now explain skips in plain language (e.g. UpdateCount was 0) and truncate long captures so misconfigured apt listings cannot flood the UI.

service/internal/runner/vars_test.go

@@ -0,0 +1,33 @@
+package runner
+
+import "testing"
+
+func TestValidateVariableName(t *testing.T) {
+	if err := ValidateVariableName("UpdateCount"); err != nil {
+		t.Fatalf("UpdateCount: %v", err)
+	}
+	if err := ValidateVariableName("_x"); err != nil {
+		t.Fatalf("_x: %v", err)
+	}
+	if err := ValidateVariableName(""); err == nil {
+		t.Fatal("expected empty name error")
+	}
+	if err := ValidateVariableName("1bad"); err == nil {
+		t.Fatal("expected invalid name error")
+	}
+	if err := ValidateVariableName("bad-name"); err == nil {
+		t.Fatal("expected invalid name error")
+	}
+}
+
+func TestExpandDollarVars(t *testing.T) {
+	vars := map[string]string{"UpdateCount": "3", "Name": "web"}
+	got := ExpandDollarVars("count=$UpdateCount host=${Name}-01 keep=$HOME", vars)
+	want := "count=3 host=web-01 keep=$HOME"
+	if got != want {
+		t.Fatalf("got %q want %q", got, want)
+	}
+	if ExpandDollarVars("$Missing", nil) != "$Missing" {
+		t.Fatal("nil vars should leave tokens unchanged")
+	}
+}