Fix OpenAI multi-turn failures by only setting store: false in Codex mode

The Codex endpoint commit (fd9260c3) added `store: false` unconditionally
to all OpenAI Responses API requests. This broke multi-turn conversations
because OpenAI doesn't persist items when store is false, so referencing
previous reasoning/message IDs on subsequent turns returns a 404. The fix
makes store conditional: true for regular OpenAI (the API default), false
only for the Codex endpoint which requires it.

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

lib/crates/fabro-llm/src/providers/openai.rs

@@ -414,7 +414,7 @@ fn build_api_request(request: &Request, stream: bool, codex_mode: bool) -> ApiRe
         text,
         stop: request.stop_sequences.clone(),
         metadata: request.metadata.clone(),
-        store: false,
+        store: !codex_mode,
         stream,
     }
 }