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/dags_migrated/example.yaml

@@ -0,0 +1,23 @@
+# Example Dagu DAG
+# This is a simple example workflow
+
+name: example
+description: Example workflow to demonstrate Dagu capabilities
+
+schedule:
+  # Run every day at 9:00 AM
+  - "0 9 * * *"
+
+steps:
+  - name: hello
+    command: echo "Hello from Dagu!"
+
+  - name: list_files
+    command: ls -la /home/lucas/dagu/scripts
+    depends:
+      - hello
+
+  - name: date
+    command: date
+    depends:
+      - hello