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

internal/apps/edge/config/duration_test.go

@@ -0,0 +1,53 @@
+package config
+
+import (
+	"encoding/json"
+	"testing"
+	"time"
+)
+
+func TestMillisecondDurationUnmarshalJSON(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		want    time.Duration
+		wantErr bool
+	}{
+		{name: "null", input: "null", want: 0},
+		{name: "empty", input: `""`, want: 0},
+		{name: "integer milliseconds", input: "30000", want: 30 * time.Second},
+		{name: "duration string", input: `"5s"`, want: 5 * time.Second},
+		{name: "invalid number", input: "not-a-number", wantErr: true},
+		{name: "invalid duration string", input: `"not-a-duration"`, wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var d MillisecondDuration
+			err := json.Unmarshal([]byte(tt.input), &d)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatal("expected error")
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if d.Duration() != tt.want {
+				t.Fatalf("got %s, want %s", d, tt.want)
+			}
+		})
+	}
+}
+
+func TestMillisecondDurationMarshalJSON(t *testing.T) {
+	d := MillisecondDuration(7 * time.Second)
+	data, err := json.Marshal(d)
+	if err != nil {
+		t.Fatalf("Marshal failed: %v", err)
+	}
+	if string(data) != "7000" {
+		t.Fatalf("unexpected marshaled value: %s", string(data))
+	}
+}