refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/infra/storage/objectstore/local_test.go

@@ -0,0 +1,51 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package objectstore
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"testing"
+)
+
+func TestLocalBackendRoundTrip(t *testing.T) {
+	backend, err := newLocalBackend(LocalConfig{Root: t.TempDir()})
+	if err != nil {
+		t.Fatalf("newLocalBackend() returned error: %v", err)
+	}
+	ctx := context.Background()
+	const key = "uploads/2026/06/13/test.txt"
+	const content = "wavelet storage"
+
+	storedResult, err := backend.Put(ctx, key, bytes.NewBufferString(content), int64(len(content)), "text/plain")
+	if err != nil {
+		t.Fatalf("Put(%q) returned error: %v", key, err)
+	}
+	if storedResult.Key != key {
+		t.Errorf("Put(%q) key = %q, want %q", key, storedResult.Key, key)
+	}
+
+	object, err := backend.Get(ctx, key)
+	if err != nil {
+		t.Fatalf("Get(%q) returned error: %v", key, err)
+	}
+	got, err := io.ReadAll(object.Body)
+	if err != nil {
+		t.Fatalf("ReadAll(Get(%q)) returned error: %v", key, err)
+	}
+	if err := object.Body.Close(); err != nil {
+		t.Fatalf("Close(Get(%q)) returned error: %v", key, err)
+	}
+	if string(got) != content {
+		t.Errorf("Get(%q) content = %q, want %q", key, got, content)
+	}
+
+	if err := backend.Delete(ctx, key); err != nil {
+		t.Fatalf("Delete(%q) returned error: %v", key, err)
+	}
+	if _, err := backend.Get(ctx, key); err == nil {
+		t.Errorf("Get(%q) after Delete() returned nil error", key)
+	}
+}