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/src/environment_context.rs

@@ -28,6 +28,7 @@ impl EnvironmentContext {
             cwd,
             // should compare all fields except shell
             shell: _,
+            ..
         } = other;
 
         self.cwd == *cwd