fix(system): remove .aicentral/ — canonical dir is .ai_central/

The memory_bank.central.json was committed to .aicentral/ (no underscore)
but the canonical directory used by ai_mail and flow is .ai_central/
(with underscore). Removing the wrong one — .ai_central/ is gitignored
and managed locally like all central aggregation files.

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

.aicentral/memory_bank.central.json

@@ -1,9 +0,0 @@
-{
-  "service": "memory_bank",
-  "last_updated": "2026-03-17T18:19:25.268718",
-  "stats": {
-    "total_vectors": 0,
-    "total_archives": 0,
-    "last_rollover": ""
-  }
-}