Back out "feat: POSIX unification and snapshot sessions (#3179)" (#3430)

This reverts https://github.com/openai/codex/pull/3179.

#3179 appears to introduce a regression where sourcing dotfiles causes a
bunch of activity in the title bar (and potentially slows things down?)


https://github.com/user-attachments/assets/a68f7fb3-0749-4e0e-a321-2aa6993e01da

Verified this no longer happens after backing out #3179.

Original commit changeset: 62bd0e3d9d

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

@@ -17,7 +17,6 @@ use core_test_support::load_default_config_for_test;
 use core_test_support::load_sse_fixture_with_id;
 use core_test_support::wait_for_event;
 use tempfile::TempDir;
-use uuid::Uuid;
 use wiremock::Mock;
 use wiremock::MockServer;
 use wiremock::ResponseTemplate;
@@ -270,7 +269,7 @@ async fn prefixes_context_and_instructions_once_and_consistently_across_requests
     let requests = server.received_requests().await.unwrap();
     assert_eq!(requests.len(), 2, "expected two POST requests");
 
-    let shell = default_user_shell(Uuid::new_v4(), codex_home.path()).await;
+    let shell = default_user_shell().await;
 
     let expected_env_text = format!(
         r#"<environment_context>