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.go

@@ -0,0 +1,54 @@
+package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type MillisecondDuration time.Duration
+
+func (d MillisecondDuration) Duration() time.Duration {
+	return time.Duration(d)
+}
+
+func (d MillisecondDuration) String() string {
+	return time.Duration(d).String()
+}
+
+func (d *MillisecondDuration) UnmarshalJSON(data []byte) error {
+	raw := strings.TrimSpace(string(data))
+	if raw == "" || raw == "null" {
+		*d = 0
+		return nil
+	}
+	if strings.HasPrefix(raw, "\"") {
+		var text string
+		if err := json.Unmarshal(data, &text); err != nil {
+			return err
+		}
+		text = strings.TrimSpace(text)
+		if text == "" {
+			*d = 0
+			return nil
+		}
+		parsed, err := time.ParseDuration(text)
+		if err != nil {
+			return fmt.Errorf("invalid duration string %q: %w", text, err)
+		}
+		*d = MillisecondDuration(parsed)
+		return nil
+	}
+	ms, err := strconv.ParseInt(raw, 10, 64)
+	if err != nil {
+		return fmt.Errorf("invalid duration milliseconds %q: %w", raw, err)
+	}
+	*d = MillisecondDuration(time.Duration(ms) * time.Millisecond)
+	return nil
+}
+
+func (d MillisecondDuration) MarshalJSON() ([]byte, error) {
+	return json.Marshal(time.Duration(d).Milliseconds())
+}

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))
+	}
+}