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/heartbeat/loop_test.go

@@ -0,0 +1,41 @@
+package heartbeat
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestRunLoopImmediateAndTicker(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	var calls atomic.Int32
+	interval := 20 * time.Millisecond
+
+	done := make(chan struct{})
+	go func() {
+		RunLoop(ctx, interval, func(context.Context) {
+			calls.Add(1)
+		})
+		close(done)
+	}()
+
+	time.Sleep(5 * time.Millisecond)
+	if got := calls.Load(); got != 1 {
+		t.Fatalf("expected 1 immediate call, got %d", got)
+	}
+
+	time.Sleep(35 * time.Millisecond)
+	if got := calls.Load(); got < 2 {
+		t.Fatalf("expected at least 2 calls after ticker, got %d", got)
+	}
+
+	cancel()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("RunLoop did not exit after context cancellation")
+	}
+}