conductor(tracks): Add two deferred future tracks

- aggregation_smarter_summaries: Sub-agent summarization, hash-based caching
- system_context_exposure: Expose hidden _SYSTEM_PROMPT for user customization

conductor/tracks/aggregation_smarter_summaries_20260322/metadata.json

@@ -0,0 +1,17 @@
+{
+  "name": "aggregation_smarter_summaries",
+  "created": "2026-03-22",
+  "status": "future",
+  "priority": "medium",
+  "affected_files": [
+    "src/aggregate.py",
+    "src/file_cache.py",
+    "src/ai_client.py",
+    "src/models.py"
+  ],
+  "related_tracks": [
+    "discussion_hub_panel_reorganization (in_progress)",
+    "system_context_exposure (future)"
+  ],
+  "notes": "Deferred from discussion_hub_panel_reorganization planning. Improves aggregation with sub-agent summarization and hash-based caching."
+}