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/agent/logging/logger.go

@@ -1,29 +1,7 @@
 package logging
 
-import (
-	"log/slog"
-	"os"
-	"strings"
-)
+import edgelogging "github.com/Rain-kl/Wavelet/internal/apps/edge/logging"
 
 func Setup() {
-	opts := &slog.HandlerOptions{
-		AddSource: true,
-		Level:     parseLevel(os.Getenv("LOG_LEVEL")),
-	}
-	handler := slog.NewTextHandler(os.Stdout, opts)
-	slog.SetDefault(slog.New(handler))
-}
-
-func parseLevel(value string) slog.Level {
-	switch strings.ToLower(strings.TrimSpace(value)) {
-	case "debug":
-		return slog.LevelDebug
-	case "warn", "warning":
-		return slog.LevelWarn
-	case "error":
-		return slog.LevelError
-	default:
-		return slog.LevelInfo
-	}
-}
+	edgelogging.Setup(edgelogging.Options{AddSource: true})
+}