chore: clear auto-chain flag after phase execution

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

Author: JasperSeehofer

.planning/config.json

@@ -9,6 +9,6 @@
     "plan_check": true,
     "verifier": true,
     "nyquist_validation": true,
-    "_auto_chain_active": true
+    "_auto_chain_active": false
   }
 }