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/go.mod

@@ -5,6 +5,7 @@ go 1.25.0
 require (
 	fn-registry v0.0.0-00010101000000-000000000000
 	github.com/mattn/go-sqlite3 v1.14.37
+	nhooyr.io/websocket v1.8.17
 )
 
 require (
@@ -28,19 +29,21 @@ require (
 	github.com/marcboeker/go-duckdb v1.8.5 // indirect
 	github.com/paulmach/orb v0.12.0 // indirect
 	github.com/pierrec/lz4/v4 v4.1.25 // indirect
-	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	github.com/segmentio/asm v1.2.1 // indirect
 	github.com/shopspring/decimal v1.4.0 // indirect
 	github.com/zeebo/xxh3 v1.0.2 // indirect
 	go.opentelemetry.io/otel v1.41.0 // indirect
 	go.opentelemetry.io/otel/trace v1.41.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
+	golang.org/x/crypto v0.50.0 // indirect
 	golang.org/x/exp v0.0.0-20250128182459-e0ece0dbea4c // indirect
-	golang.org/x/mod v0.27.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
-	golang.org/x/sys v0.41.0 // indirect
-	golang.org/x/text v0.29.0 // indirect
-	golang.org/x/tools v0.36.0 // indirect
+	golang.org/x/mod v0.34.0 // indirect
+	golang.org/x/net v0.53.0 // indirect
+	golang.org/x/sync v0.20.0 // indirect
+	golang.org/x/sys v0.43.0 // indirect
+	golang.org/x/telemetry v0.0.0-20260311193753-579e4da9a98c // indirect
+	golang.org/x/text v0.36.0 // indirect
+	golang.org/x/tools v0.43.0 // indirect
 	golang.org/x/xerrors v0.0.0-20240903120638-7835f813f4da // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )