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.go

@@ -0,0 +1,23 @@
+package heartbeat
+
+import (
+	"context"
+	"time"
+)
+
+// RunLoop invokes fn immediately, then on each interval tick until ctx is cancelled.
+func RunLoop(ctx context.Context, interval time.Duration, fn func(context.Context)) {
+	fn(ctx)
+
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			fn(ctx)
+		}
+	}
+}