refactor(task): replace init registration with bootstrap wiring

Introduce internal/bootstrap as the composition root with sync.Once
guards for task handler registration and push listener wiring. Replace
the single OnTaskCompleted global hook with multi-subscriber handlers
and remove init()-driven side effects from worker, admin task, and push.

internal/cmd/all.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"sync"
 
+	"github.com/Rain-kl/Wavelet/internal/bootstrap"
 	"github.com/Rain-kl/Wavelet/internal/router"
 	"github.com/Rain-kl/Wavelet/internal/task/scheduler"
 	"github.com/Rain-kl/Wavelet/internal/task/worker"
@@ -20,6 +21,7 @@ var allCmd = &cobra.Command{
 	Short: "以融合模式同时启动 API、Worker 和 Scheduler",
 	Run: func(_ *cobra.Command, _ []string) {
 		log.Println("[All] 融合模式启动")
+		bootstrap.RegisterAll()
 
 		var wg sync.WaitGroup