feat: add DAG engine issues (0007a-e) and feature flag

Desglose del sistema de orquestacion propio para reemplazar Dagu:
- 0007a: core puro (parse, validate, topo sort)
- 0007b: process manager (spawn, wait, kill)
- 0007c: execution store (SQLite)
- 0007d: scheduler (cron parser, ticker)
- 0007e: app CLI que compone todo

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dev/feature_flags.json

@@ -0,0 +1,7 @@
+{
+  "dag-engine": {
+    "enabled": false,
+    "issue": "0007",
+    "description": "Sistema propio de orquestacion de DAGs para reemplazar Dagu. Incluye parser YAML, executor con paralelismo, process manager, execution store y scheduler cron."
+  }
+}