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/pkg/util/like_test.go

@@ -0,0 +1,22 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import "testing"
+
+func TestEscapeLike(t *testing.T) {
+	cases := map[string]string{
+		"":            "",
+		"/my_page":    `/my\_page`,
+		"100%":        `100\%`,
+		`a\b`:         `a\\b`,
+		`%_\`:         `\%\_\\`,
+		"normal/path": "normal/path",
+	}
+	for input, want := range cases {
+		if got := EscapeLike(input); got != want {
+			t.Errorf("EscapeLike(%q) = %q, want %q", input, got, want)
+		}
+	}
+}