chore(conductor): Add missing index.md and metadata.json to new tracks

conductor/tracks/mma_data_architecture_dag_engine/index.md

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+# MMA Data Architecture & DAG Engine
+
+Restructures manual_slop state and execution into a per-track DAG model.
+
+### Navigation
+- [Specification](./spec.md)
+- [Implementation Plan](./plan.md)

conductor/tracks/mma_data_architecture_dag_engine/metadata.json

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+{
+  "id": "mma_data_architecture_dag_engine",
+  "title": "MMA Data Architecture & DAG Engine",
+  "status": "planned",
+  "created_at": "2026-02-27T19:20:00.000000"
+}