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/issues/README.md

@@ -0,0 +1,15 @@
+# Issues
+
+| ID | Título | Estado | Prioridad | Tipo | Bloquea |
+|----|--------|--------|-----------|------|---------|
+| 0001 | Jupyter create notebook | completado | — | feature | — |
+| 0002 | Jupyter discover root dir | completado | — | bugfix | — |
+| 0003 | Jupyter tools documentation | completado | — | docs | — |
+| 0004 | Jupyter discover multiple instances | completado | — | feature | — |
+| 0005 | Jupyter write batch | completado | — | feature | — |
+| 0006 | Jupyter exec outputs keyerror | completado | — | bugfix | — |
+| **0007a** | **DAG engine: core (parse, validate, topo sort)** | pendiente | alta | feature | 0007b-e |
+| **0007b** | **DAG engine: process manager (spawn, wait, kill)** | pendiente | alta | feature | 0007e |
+| **0007c** | **DAG engine: execution store (SQLite)** | pendiente | alta | feature | 0007e |
+| **0007d** | **DAG engine: scheduler (cron parser, ticker)** | pendiente | media | feature | 0007e |
+| **0007e** | **DAG engine: app CLI que reemplaza Dagu** | pendiente | alta | feature | — |