Add 3-Day Outlook generation

internal/promptinput/package.go

@@ -107,8 +107,8 @@ func Validate(pkg Package) error {
 	if pkg.Briefing.Metadata.ReportID != pkg.Report.ID {
 		return fmt.Errorf("briefing.metadata.reportId must match report.id")
 	}
-	if pkg.Briefing.Daily == nil {
-		return fmt.Errorf("briefing.daily is required")
+	if pkg.Briefing.Daily == nil && pkg.Briefing.ThreeDay == nil {
+		return fmt.Errorf("briefing report content is required")
 	}
 	return nil
 }

internal/promptinput/package_test.go

@@ -52,6 +52,29 @@ func TestValidateRequiresFields(t *testing.T) {
 	}
 }
 
+func TestBuildThreeDayDataPackage(t *testing.T) {
+	briefingPackage := validBriefingPackage()
+	briefingPackage.Metadata.RunID = "20260529T100000Z_three_day"
+	briefingPackage.Metadata.ReportID = report.ThreeDay
+	briefingPackage.Metadata.PromptID = "weather.three_day_outlook"
+	briefingPackage.Daily = nil
+	briefingPackage.ThreeDay = &briefing.ThreeDay{
+		Days: []briefing.OutlookDay{{Date: "2026-05-29"}},
+	}
+
+	pkg, err := Build(briefingPackage)
+	if err != nil {
+		t.Fatalf("Build() error = %v", err)
+	}
+
+	if pkg.Report.ID != report.ThreeDay {
+		t.Fatalf("Report.ID = %q, want three_day", pkg.Report.ID)
+	}
+	if pkg.Briefing.ThreeDay == nil {
+		t.Fatal("Briefing.ThreeDay = nil")
+	}
+}
+
 func TestMarshalDeterministic(t *testing.T) {
 	pkg, err := Build(validBriefingPackage())
 	if err != nil {