refactor(edge): 抽取边缘运行时共享包并完成 Phase 3 重构

- 新增 internal/apps/edge/，三组件改为薄包装，删除 3000+ 行重复代码
- Agent 心跳周期下沉至 heartbeat/cycle.go
- 协议类型迁入 pkg/protocol/agent.go
- 补充设计文档与 changelog

pkg/protocol/agent_test.go

@@ -0,0 +1,95 @@
+package protocol
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestAgentProtocolJSONTags(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name     string
+		value    any
+		expected map[string]string
+	}{
+		{
+			name:  "NodePayload",
+			value: NodePayload{},
+			expected: map[string]string{
+				"NodeID": "node_id",
+				"Name":   "name",
+			},
+		},
+		{
+			name:  "AgentSettings",
+			value: AgentSettings{},
+			expected: map[string]string{
+				"HeartbeatInterval":       "heartbeat_interval",
+				"WebsocketUpgradeEnabled": "websocket_upgrade_enabled",
+				"RestartOpenrestyNow":     "restart_openresty_now",
+			},
+		},
+		{
+			name:  "WSMessage",
+			value: WSMessage{},
+			expected: map[string]string{
+				"Type":    "type",
+				"Payload": "payload,omitempty",
+			},
+		},
+		{
+			name:  "RegisterNodeResponse",
+			value: RegisterNodeResponse{},
+			expected: map[string]string{
+				"NodeID":      "node_id",
+				"AccessToken": "agent_token",
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			typ := reflect.TypeOf(tc.value)
+			for field, wantTag := range tc.expected {
+				structField, ok := typ.FieldByName(field)
+				if !ok {
+					t.Fatalf("field %q not found on %s", field, tc.name)
+				}
+				gotTag := structField.Tag.Get("json")
+				if gotTag != wantTag {
+					t.Fatalf("field %q json tag = %q, want %q", field, gotTag, wantTag)
+				}
+			}
+		})
+	}
+}
+
+func TestNodePayloadJSONRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	payload := NodePayload{
+		NodeID:          "node-1",
+		Name:            "edge-a",
+		OpenrestyStatus: OpenrestyStatusHealthy,
+		HealthEvents:    []NodeHealthEvent{},
+	}
+
+	encoded, err := json.Marshal(payload)
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+
+	var decoded NodePayload
+	if err := json.Unmarshal(encoded, &decoded); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+
+	if decoded.NodeID != payload.NodeID || decoded.Name != payload.Name {
+		t.Fatalf("round trip mismatch: %+v", decoded)
+	}
+}