fix: disable fidelity content replacement to stop rapid retry loops

L1/L2 stub replacement was producing responses that opencode
rejected, triggering rapid retries and rate limiting. Disabled
until stub format is validated for tool_result compatibility.

src/mnemosyne/gateway.py

@@ -2183,7 +2183,9 @@ def create_app(
         #     )
 
         # 4b. Apply fidelity-based content replacement
-        _apply_fidelity(payload, session)
+        # DISABLED: L1/L2 stub replacement causes response issues that trigger
+        # rapid retries in opencode. Re-enable once stub format is validated.
+        # _apply_fidelity(payload, session)
 
         # Place cache_control markers at optimal positions
         _place_cache_controls(payload)