fix: disable _apply_fidelity again — stubs cause rapid retry loops

Second time disabling. The auto-stub replacement produces content
that opencode rejects, triggering 16+ retries per second. Needs
deeper investigation into which content types can be safely stubbed
vs which ones break the model.

src/mnemosyne/gateway.py

@@ -2303,7 +2303,9 @@ def create_app(
         #     )
 
         # 4b. Apply fidelity-based content replacement
-        _apply_fidelity(payload, session)
+        # DISABLED: stubs break model responses causing rapid retry loops.
+        # Needs investigation into what content types are safe to stub.
+        # _apply_fidelity(payload, session)
 
         # Place cache_control markers at optimal positions
         _place_cache_controls(payload)