feat(dag_engine): WS hub /api/ws/dagruns + migracion DAGs desde dagu

- events.go: DagRunHub broadcastea snapshot+deltas live (500ms tick, 5s recent finished window) sobre dag_runs + dag_step_results.
- api.go: handler GET /api/ws/dagruns upgrade WS, opt-in en RegisterAPI.
- store.go: expone Conn() para read-only desde el hub.
- main.go: construye DagRunHub al arrancar server.
- dags_migrated/: 5 YAMLs migrados desde ~/dagu/dags tras desinstalar dagu (issue 0095 step 1).

Smoke: snapshot inicial OK, trigger /api/dags/test_claude_access/run -> delta WS observa 3 step_results + run success en <1s.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/dag_engine/api.go

@@ -6,7 +6,7 @@ import (
 )
 
 // RegisterAPI sets up all HTTP routes on the given mux.
-func RegisterAPI(mux *http.ServeMux, executor *Executor, scheduler *Scheduler, frontendFS fs.FS) {
+func RegisterAPI(mux *http.ServeMux, executor *Executor, scheduler *Scheduler, hub *DagRunHub, frontendFS fs.FS) {
 	// API routes.
 	mux.HandleFunc("GET /api/dags", handleListDags(executor))
 	mux.HandleFunc("GET /api/dags/{name}", handleGetDag(executor))
@@ -19,6 +19,11 @@ func RegisterAPI(mux *http.ServeMux, executor *Executor, scheduler *Scheduler, f
 	mux.HandleFunc("POST /api/scheduler/stop", handleSchedulerStop(scheduler))
 	mux.HandleFunc("GET /api/scheduler/status", handleSchedulerStatus(scheduler))
 
+	// Live updates (WS hub).
+	if hub != nil {
+		mux.HandleFunc("GET /api/ws/dagruns", handleDagRunsWS(hub))
+	}
+
 	// Frontend SPA fallback.
 	if frontendFS != nil {
 		mux.Handle("/", spaHandler(frontendFS))