feat: POSIX unification and snapshot sessions (#3179)

## Session snapshot
For POSIX shell, the goal is to take a snapshot of the interactive shell
environment, store it in a session file located in `.codex/` and only
source this file for every command that is run.
As a result, if a snapshot files exist, `bash -lc <CALL>` get replaced
by `bash -c <CALL>`.

This also fixes the issue that `bash -lc` does not source `.bashrc`,
resulting in missing env variables and aliases in the codex session.
## POSIX unification
Unify `bash` and `zsh` shell into a POSIX shell. The rational is that
the tool will not use any `zsh` specific capabilities.

---------

Co-authored-by: Michael Bolin <mbolin@openai.com>

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

@@ -17,6 +17,7 @@ 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;
@@ -269,7 +270,7 @@ 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().await;
+    let shell = default_user_shell(Uuid::new_v4(), codex_home.path()).await;
 
     let expected_env_text = format!(
         r#"<environment_context>