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/core/manifest.go

@@ -0,0 +1,38 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package core
+
+import (
+	"fmt"
+	"strings"
+)
+
+// Manifest defines the metadata and dependency declarations for a plugin.
+type Manifest struct {
+	// Name is the unique identifier for the plugin (e.g. "auth", "user", "order").
+	Name string `json:"name" yaml:"name"`
+
+	// Version is the semantic version string of the plugin (e.g. "1.0.0").
+	Version string `json:"version,omitempty" yaml:"version,omitempty"`
+
+	// Description gives a brief summary of the plugin capabilities.
+	Description string `json:"description,omitempty" yaml:"description,omitempty"`
+
+	// Author specifies the author or maintainer of the plugin.
+	Author string `json:"author,omitempty" yaml:"author,omitempty"`
+
+	// Dependencies lists the plugin names that this plugin depends on.
+	Dependencies []string `json:"dependencies,omitempty" yaml:"dependencies,omitempty"`
+
+	// Metadata holds arbitrary plugin-specific metadata.
+	Metadata map[string]any `json:"metadata,omitempty" yaml:"metadata,omitempty"`
+}
+
+// Validate checks whether the manifest satisfies basic integrity requirements.
+func (m Manifest) Validate() error {
+	if strings.TrimSpace(m.Name) == "" {
+		return fmt.Errorf("%w: %w", ErrInvalidManifest, ErrInvalidManifestName)
+	}
+	return nil
+}