Harden configuration validation

internal/app/session_cli_test.go

@@ -100,6 +100,26 @@ inputs:
 	}
 }
 
+func TestPlanRequiresExpectedPreviousSessionID(t *testing.T) {
+	workspaceRoot := t.TempDir()
+	pipelinePath, campaignPath, sessionPath := writeValidConfigFiles(t, workspaceRoot)
+
+	var out bytes.Buffer
+	err := Plan(context.Background(), []string{
+		"2026-05-03",
+		"--config", pipelinePath,
+		"--campaign-file", campaignPath,
+		"--session", sessionPath,
+		"--previous-session-id", "2026-04-26",
+	}, &out)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "previous_session_id is required") {
+		t.Fatalf("error = %q, want missing previous-session expectation", err.Error())
+	}
+}
+
 func TestRunStageAcceptsPositionalSessionIDAndParsesStageName(t *testing.T) {
 	workspaceRoot := t.TempDir()
 	pipelinePath, campaignPath, sessionPath := writeValidConfigFiles(t, workspaceRoot)