make codex better at git (#10145)

adds basic git context to the session prefix so the model can anchor git
actions and be a bit more version-aware. structured it in a
multiroot-friendly shape even though we only have one root today

codex-rs/core/tests/suite/client.rs

@@ -1190,7 +1190,7 @@ async fn azure_responses_request_includes_store_and_reasoning_ids() {
         SessionSource::Exec,
         TransportManager::new(),
     )
-    .new_session();
+    .new_session(None);
 
     let mut prompt = Prompt::default();
     prompt.input.push(ResponseItem::Reasoning {